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Chapter XX. The Skeleton

The literature of the skeleton is very extensive as regards both its development and comparative anatomy. The ease with which skeletons can 1)0 prepared and the imix)rtance of the hard parts to the palaeontologist has long given the skeleton a prominence in morphological research far in excess of its importance as compared with the other systems. Athough the skeleton is in the mexhanical sense the framework of the body, it is not so in the morphological sense, because so far is it from being the framework \x\yoxi which the body is built up, that its development is entirely subsidiary" to the development of other systems, and is dominated by the arrangement of other tissues and organs, which have been fonned and arranged before the skeleton even begins to appear.

In this chapter there is no attempt to give an exhaustive treatise upon the development, but by following the summaries given bv Kolliker (*'Entwickclungsgeschichte," t>te Autl., 401-502), Hertwig ("Mirbuch," 3te Aufl., 41)2-r43), and W. K. Parker (** Morphology of the Skull ), and consultation of the more imix)rtant original authorities, I have endeavore<l to write a comprehensive account of the subject.

Stages of the Skeleton. — We must distinguish between the stages of the skeleton as a whole, and the stages in the histogenesis of the bones. It must also l>e constantly bonie in mind that the vertebrates have two morphologically distinct skeletons, the primarA* cartilaginous skeleton, which in tho higher forms becomes partly ossified, and the secondary skeleton composed of dermal bones.

Notochonlal Structure, — Permanent in amphioxus. In this stage the only skeleton is the axial rod of the notochortl, and it is found to l)e the first stage in all A'ertebrate embryos.

The Membranous Stage. — The second stage in all true vertebrates and tho permanent one in marsipobranchs. the mesenchyma is condensed around the notochord and strengthens thus the axis.

The Primary Cartilaginous Stage. — The principal parts of the primary skeleton and representtnl by separate cartilages.

The Completed Cartilaginous Stage. — In which all the parts of the primary skeleton are present in the form of cartilages. No definite line can be drawn ljetwiH.'n this stage and the j)rece(ling, nor between it and the following.

Stage of the dermal skeleton, characterized by the development of sundry bones in the dermis. Dermatomes begin to develop before the cartilages ossify, and are present in cnrtilaginous fishes, hence they must In? considennl as old(»r, and therefore Ix'longing to an earlier stage, than the l>ones n»placing cartilages.

Stage with osseous primary skeleton, characterized by the primary cartilages being replaced by bone. The replacement is very gradual and never becomes complete; it begins in some of the cartilages before others are developed ; it is, accordingly, impossible to establish any definite limit in time for this stage.

The most logical treatment would be to deal with these six stages in their natural sequence, but it has appeared to me more convenient to give the complete history of the notochord by itself (see p. 181), to add a section upon the membranous stage, and then to present the entire history of the primary skeleton under two main heads, the axial skeleton, p. 424, and the appendicular, p. 448; leaving the dermal skeleton till the last, p. 461, although it is ontogcnetically and phylogenetically older than the osseous primary skeleton. The chapter doses, p. 4G5, with some general remarks on the morphology of the skull.

Membranous Stage

As we have already seen, the mesothelium of the inner side of the primitive segments produces the mesenchymal cells, which invest the notochord and medullary canal. Recent writers have tended to regard this periaxial mesenchyma as segmented, and Van Wijhe even proposes to bestow the special name of sklerotome upon each of the mesenchymal segments. It is true that owing to its segmented origin the tissue does show for a time traces of metameric division, but the division becomes unrecognizable long before there is any mesenchymal skeleton indicated. The primary segmentation plays no immediate part in the development of the separate vertebrae. These considerations render it unjustifiable to regard the periaxial mesenchj^ma as segmented. We ought not to speak of sklerotomes unless we are prepared to speak of dermotomes, because the anlage of the dermal mesenchjTna is as much segmented as the anlage of the periaxial mesenchyma. The question under consideration arose from a mistake of the older embryologists, who believed that the primitive segments were the direct anlages of the vertebra?, and accordingly named them protovertebrse ( Urwtrbel) ; unfortunately this misleading term is still in use. Then came the discovery that the true vertebrae are developed apparently between the primitive segments or in alternation with them. Remak formulated the hypothesis of resegmentation of the skeleton {Xevgliedenuig des Axenskeletki), which is wrong in assuming that the segmentation of the skeleton is not parallel with the primary segments, but is right in assuming that there is a primary segmenbition of the skeleton, corresponding to the original mesothelial segments. Remak's conception luis perpetuated itself to this day, and is carefully rej^eated in current text-books ; were it correct in its entirety then the memhranouH stage we are now considering would not occur.

The first step toward the development of the perichordal skeleton is the fusion of the loose mesenchyma, derived from the segmented mesotheliuni, into a continuous mass of cells, which grow around the not<K?hord and se|)arato it first from the entoderm and later from the medullary canal, and grow around the medullary canal and close over it slowly, and also grow around the primitive aort^e, see Figs. IGl and 10:5. This mesenchyma is of a loose but not (juite uniform character, and the cells earlj' begin to condense in the immediate neighborhod of the notochord and nervous system. Around the uotochord the cells gradually become very closelj'^ crowded and form what is known in the lower vertebrates as the chorda sheath, in the amniote embryo as the investing mai?s, but in the anmiota the uniform continuous sheath exists only around the anterior end of the notochonl where the investing mass participates in the formation of the cranium, while throughout the remainder of the embryo, as has been shown by A. Fnjrit^p, the condensed mesenchjTnal anlage is divided from the start more or less distinctly into separate vertebral masses, which in staineil sections stand out conspicuously. Froriep luis studied the development of the vertebra) in the chick, 83.1, and mammals (cow embryos), 86.1,

I. Axial Skeleton

Vertebral Column

As to how far forward the vertebral column extends in the head we have no means of deciding positively, but as the occipital region of the skull is developed by the fusion of vertebrae, and as these vertebra? appear less and less distinctly as we pass forward from the neck, anci as the number of occipital vertebrae is greater in birds than in mammals, we cannot avoid the supi)osition that the number of vertebrae fused in the head was once grt^ter than now appears in the mammalian embryo. There is accordingly much uncertainty as to the number of cephalic vertebrse. But though the numl>er of vertebne is not exactly known, wo can fix iho position of the cephalic end of the vertebral colunm, jis coincident with the cephalic end of the notochord, which is close to the hy|)ophysis or pituitary body. The notochord bo(*omes investetl almost up to its ceplialic extremit}^ by the condensed mesenchymal sheath, which is found in the occij)ital region, as in the body, to bo the bliistema out of which are differentiated the anlag(^s of the vertebrae; it api^ears, therefore, no mere imagination to regard this as homologous with the vertebral column throughout, but with the development of the vertebrie inhibited entirely in the anterior, partially in the ])osterior occipital region. In front of the pituitary body the not<x*hord and consequently the invi»sting mass do not extend. We must in fact divide the head into a pra*-]>ituitary unvertebrated an<l a i)ost -pituitary vertebra ted region. the remaining vertebra) to the end of the tail develop all much alike. They iissume, however, nKHliHtnl forms in the various regions, but the origin in the embryo of the differences betwecm the cervical, dorsal, and lumKir vertebne has never Ijeen worked out. Special modifications of the first and st^cond cervical vertebrae take place in mammals to form the atlas and epistropheus c»r axis, m the five sacral verti»l>rae to fonn the sacrum, and in the caudal vertebne to fonn the c<x\yx.

Typical Development of a Vertebra.— Our exact knowledge rests mainly ujxm the investigations of August Froriep, 83.1, 86. 1, on chick and cow embr\x>s. The investing mass or condenseil lx?richonlal mesenchyma forms a continuous sheath around the notochord. At a pi'^int (MrresiH>n<ling to the centn^ of each mesodermic segment, or a little on the cephalic side of each st^gment, the investing mass becomes thicker in diameter and it« tissue more condensed ; the condensation is verj- noticeable in stained sections and is the first sign of the vertebral fonuation ; the condensHtion spreads rapidly, extending sideways, upward, and backward with the result of forming a Ik)w of dense mesenchj-ma, the primitive vertebral bow (Wirbelliogeit) of Froriep. These bows are distinct from the bodies of the vertebrse, which arise lat«r from separate anlages. The bows pass on the ventral side of the notochord. and thence arch on each side, Fig. 9-i:J, tailward and dorealward, so as to end at the caudal edge of the muscle plate of the segment to which they belong, and ending, therefore, just in front of the intersegmental arterj-, v. and of the p nal uer e X from the sensor gangl on of tl e next f Uowing seg ne t We see here that the ertel ne are stnctly segmental struc res '^nd ot mtersegmental as has been commo Ij assumed since

Reinak. The course of the bow, as compared with the transverse phiiie of tho body of the enibrj-o, in oblique, so that while the centre of the bow next the notochord is neai- the centre of the segment, the tips of the bow lie at the caudal limit of the segment and ultimately spparate the muscle plate of its own segment from that of the next following. The obliquity of the bow appears ti' me to be determined primarily by the arrangement of the spinal ganglia, the dorsal ends of which fill out the wi.dth of the segment, while the lower ]K>inted end is carrie<l forward to the anterior border of the segment; this disposition leaves the caudal side of the segment free for the mesenchyma and the differentiation of the vertebral bow; the obliquity is further assisted by the form of the muscle plate, as can be seen in Fig, -IVi. The [wrtiou of the bow imdemeath the chorda in the me<lian line is temted the hypoc'horda] brace (SiKiiige) and in its xdtimatc development differs considerably from the rest of the bow. The investing mass around the notiHrhord on the caudal side of the bow and above it becomes later the anl^e of the body of the vertebra. The vertebral bow may be Tegaraed as the primitive stage; it is found in the chick from the middle of the fourth to the middle of the fifth day; in cow embryos of 7-11 mm.

The vertebral bow is destined to form the processes of the vertebra, and the manner in which its ends spread out against the muscle plate can be well seen in a cross-section, Fig. 244. At the time the bow is differentiated the muscle plate has become protuberant toward the notochord, and when the dense mesenchyma forming the bow spreads out it is forced by the muscle plate to grow dorsal ward, and ventral ward, and thereby to become, as it were, branched; the dorsal branch is the anlage of the neural arch; the ventral branch the anlage of the transverse or costal process, because it grows out still farther to form the anlage of the rib.

There follows a transitional state which is characterized by the gradual development of the c^irtilaginous vertebra. This stage extends in the chick fn)ra the middle of the fifth to the middle of the sixth day, and is found in cow embryos of 12-17 mm. The notochord exliibits signs of retrogi'ossive cliange, and is contracted at the level of the vertebral bow. The part of the investing mass (perichordal mesenchvma) iinniediatelv over the centre of the bow or hypochordal brace becomes the anlage of the interrertebral ligament, its cells Ix'coming first less crowded and then acquiring an elongated form; out of tliis anlage the adult ligament is slowly differentiated, chiefly by the development of connective-tissue fibrilIsB. The investing mass behind the hy]K)chordal brace develops into the cartilaginous l>ody of the vertebra, in the mammal before, in the bird after, cartilage begins to appear in the vertebral bow. In the mammals there are two centres of chondrification, which may be recognized in the bird also, although they are in the latter connected with one another under tiie chorda. The process of chondrification continues until out of the investing mass tho separate vertebral bod^" is differentiated. Moanwhih* the chondrification goes on in the vertebral bow, and in birds the whole 1k)W is converted into cartilage and unites with tho Ijody to form the completed vertebra. In mamm^ds except in the occipital and ant^M-ior cervical vertebne the central part does not form cartilage l)ut remains as a dense mesenchymal band, which can lie recognized as a more or less distinct structure for some time, but is ultimately lost in the substance of the intervertebral ligament. A median longitudinal section of a cow embryo a little more advanced. Fig. 245, shows the persistence of the hypochordal brace.

The permanent stage is reacheil by the fusion of the cartilage of the bow with that of the body, which may be sfiid to l)e completed in the chi(*k })v the middle of the seventh dav, and in cow embrv'os of 22 nun. In the chick the whole bow is differentiated into cartilage, cUid its central part fuses with the vertebral lK)dy. In mammals this fusion does not take phice except in the occiput, but the two ends of each bow become cartilaginous and fuse with the corres|K)nding vertebral lx)dy, except in the case of the first cervical vertebra, see p. 430. The central portion of the bow in all vertebne below the first cervical disjippears and is lost in the intervertebral ligament. lu a longitudinal section, Fig. 345, it can be seen that the tirst bow is a nell developed cartilaginoiiB piece, Sc, while the second, is onlj partiall} chondnfied, while the third and fourth are almost lost m the intervertebral ligament. The first bow, as just stated, forms the atlas During the de\ elopment of the cartilage the vertebra continues gronmg and the arches extend farther from the body; the neural arches gradually close over the medullary canal, the closure taking place much earlier in the chick than in the mammal. In the human embryo the neural arches extend at eight weeks only 11 short distance up the side of the spinal cord ; at three iin mths they have come in contact on the dor sal side in tUo dorsjd region, but are still quite far apart iu the lumbar and sacral regions (Kiillikcr, " Gnindnsa," MH) and by the fourth month all the arches baveclused. The development of the spinous pnMress needs to be further investigntod. The ventral processes. Fig, ^44, spread downward and are brought, owing to the primitive inclination of thevfrt4'liral lx>w, totho caudal boundary- of the segment to which they beloiif;, and as they lie at the caudal edge of the muscle plate of their respective segments, they con- o'rui Wrv'icHi' v«tei]ni': c. f,*, b™ue» tribute to sepiirato that plate fr^.m the "} next following. These processes loose their continuity with the vertebra proper, but remain connected with it by ligaments; they thus become the independent anlages of the ribs, where true ribs are developed.

Another point deserving attention is the relation of the vertebrffi to , the vertobnil arterv which arises, as described in Chapter XXIV., as a series of l<ingitudinal anastomoses between the intersegmental arteries; the vertebral artery begins to appear in (row embryos of 12 mm,, and is ii continuous stem in those of 15 mm. The vessel forming the nna.'^toniosis grows tlinaigh the mass of the vertebral bow during the transitional stage, while the inesenchyma is not very dense at the jioint |»netratcd by the arter>\ This discovery, which we owe to Frorieji, sets aside the statement, which has l>ecome traditional, that the developing vertebra grows around the arterj", and shows instead that the artery grows thnmgh the developing anlagc of the vertebra. The artery, by its position, may te said to mark approximately the boundary between the neural and costal processes of the vertebra.

The ossificaiion of the vertebrce does not alter the morphology of the cartilaginous stage, and it is doubtful whether it is accompanied bj' any noteworthy change in the form of the single skeletal pieces. The ossification begins with two centres, one in each neural arch, and is continued by a third centre in the Ixxly of the vertebra. The centres in the neural arches lie near the body proper; that of the body appears in man about the seventh week. The centres of ossification of the l)ody become recognizable first in the dorsal region, and from there their differentiation progresses successively from vertebra to vertebra, lx)th heiidward and tailward. The centre is situated at first on the dorsal side of the chorda (Robin), but as the centre extends it incloses the notochord, which is gradually obliterated so that it can no longer lx> distinguislied aft(»r the actual formation of Ixme has commenced. The progress of ossification is very slow ; thus the preliminary degeneration covers the period, in cow embryos, in which their length increases from "2.2 to 0.0 cm., and it is not until the latter length has been atUiinod that the actual deposit of bone begins (Froriep, 86.1, 13(»). In man the centres do not attain the surface of the cartilage until the fourth or fifth month. Ultimately * the three deposits of bone fuse into a single osseous vertebra, but for a long ])eriod before this cartilage remains between the bony arches and the bony body, and on the dorsal side between the arches; these cartilaginous areas act as growing zones. The epiphyses are separate centres of ossification, whioli apjK^ar one on the cranial side, one on the caudal side of the body of each vertebra, but not until after birth. The development of tlio e])iphyses and their fusion with the main lx)dy have been investigatcKlby Schwegel, 58.1. To complete the adult bony vertebra there are five centres of ossification requisite.

Summary, — Every vertebra is developed within the limits of a single segment, that is, out of the mesenchyma produceil from the inner wall of a single segment. This point is es})ecially imjx)rtant because it is commonlv stated that each vertebra is derived from a^ljacent parts of two segments. Each vertebra has two distinct parts, -the vertebral how {Wirbelhotfen) and the vertebral lK>dy (Wirhefh'orper): both parts in their first stage consist of condenseil mesenchymal tissue. The bow appears first and is an arched band of tissue passing under the notochord, thence running obliquely backward and t(»rminating on the caudal side of the muscle plate of the segment. The lK)dy appears later in each segment just l)ehin(l the median part of the bow. The Ih)W and the Ix^dy l)otli cliondrify and fuse with one another, except in the first cervical s(»gm*^nt; in birds the whole l)ow becomes cartilaginous, but in mamniMls the middle part of the bow atrophies, except in the first cervical s(»gnient. The lateral portions of the bow form both the neural and costal arches; the distal

Eirts of the latter separate from the vertebra proper to form the anges of the ribs. The morphology of the vertebral column is completely determined while it is in the cartilaginous stage; ossification is merely a supplementary process and produces no important change in the form or anatomical relations of the vertebrae.

DiiHur the flret year after birth the arches unite dorsally, Ix'tweeu the thinl and eif^hth year the art*he« unite with the bod}'.

Froriep's discovery that the vertebral bow and body are distinct pieces must be considered very important, and at once suggests comparison with those palaeozoic reptiles in which centra and intercentra have been distinguished in the vertebral colunms, but this comparison has yet to be worked out. For a general paper on the intercentnim see Cope, 86.4, also G. Baur, 06. 1, for a discussion of the morphogeny of vertebrae from the palseontological point of view.

Evolution of Vertebrae. — We have no positive knowledge, nor even valuable theories, as to the causes which first led to the evolution of vertebrae, though unscientific hypotheses have been abundant. Thei*e is one important consideration which has been rather neglected, though almost self-evident, namely, that vertebrae have arisen within the vertebrate series, the perichordal mesenchyma in the lowest vertebrates not being divided into vertebrae, there being, in short, so-called vertebrates without vertebrae. As the higher fishes have vei'tebrae, it is evident that the vertebral column was evolved within the class of fishes.

The embryological development of the vertebrae indicates that they are compound bodies, as above shown. We are thus led to distinguish four stages in the differentiation of the axial skeleton :

Notochordal stage.

Perichordal stage.

Froriep's stage (vertebral bow and centre not united).

Vertebral stage (vertebral bow and centre united).

The first stage is permanent in Amphioxus; the second is permanent in Petromyzon ; the third will perhaps he found permanent in Chimaera ; the fourth is permanent in Amphibia and Amniota. The skull may be l(X)ked uiK)n as in part a modification of the second stage in the head region.

Occipital Vertebrae. — The occipital bone of the adult is the final outcome of the fusion and ossification of an uncertain number of vertebrae. The investing mass of the cephalic portion of the notochord forms the anlage of the occipital skeleton. This anlage terminates a short distance behind the hypophysis. In birds and mammalia it maj^ be divided into two regions, comprising each about half the length of the anlage; the anterior or pituitaiy half does not offer, even in the earliest embryonic stages, so far as known, any trace of division into separate vertebral masses; the posterior or cervical half does show clear division at an early stage into four vertebrae (in the chick into five vertebrae), but of tjfiese only the last a])pears as a perfectly distinct, well -differentiated vertebrae, but even this vertebra, when its chondrification begins, merges into the general occiyntal mass (A. Froriep, 83.1, 86.1). The vertebrae of the mammalian occiput corresjx)nd to four segments, of which the hy]M)glossns represents the nerves. Fig. 240 is a frontal projection of the cephalic end of the mesenchymal vertebral column of an embryo, 15.5 mm. long, from a cow. The nerves, N, mark the divisions between the vertebrae, as do also the intersegmental arteries, v; the anterior vertebrae are already fused, Or, but the fourth is

j^-rfectlr differentiated and cl'^irfr «imi!ar to the saceeeding verUihnb, Thr** hrf^jjflf>?iiwil nfrrveir travr^^ the ':<[xripital anlage. In emhryf^ of I'lj.o mm. th*: •oripital vertr-V.ra is foand to have fiLsed

with the tf^oi-ipital mass, thoagh the ends of it* vertebnd bow pn>j«t enough to still indicate the original division of which all trace i< ki^i in slightly older emhry.isi.

In the occipital mass chondrificatic»n Viegins on each side of the not* <rhord, wcA, just as it doe?? in th#* l* Jies of the individual vertebra-, and it begins before the fourth vertebra (Fn»riep's occipital vertebral unites with th«:t!?e in front. The result • f the chondriticatiun is to produce twu bars of cartilage which extend alongside the rjccipital notr»chord, but of o^uise,

F.a.»«.-Frr,„taiProj«.tio„..fth. a^ the hist^^i^fnetic change spreads, the Cephalic Part of a \>n*rbrai rMiunm cartilage Unites and tinally extends uiioo. la. ft mm. oiiiff through the entire anlage. The bars of

cartilage are known as the jxirarhorrlali*. and are commonly, but erroneously, <lescrilied as the primitive anlage of the occipital cranium, whereas in reality they indicate only the growth of the centres of chondrification in the anlage. I can recognize no grounds at prcisent for assigning any special moq>hological meaning to the parachordals.

Atlas and Epistropheus

The first and second cervical vertebrae undergo remarkable mcjditications, which are established during^ the transitional stage of the vertebrse — in other words, while the vertebral anlages are chondrifying. In mammals the first cervical vertebra <leveloi>s two cartilages, one of which is formetl out of the whole vertebral lx)w and giv<.*s rise to the atlas, and the other is formed out of the vertebral Ixxly. The later cartilage fuses with the sec^md vertebra and with it forms tlio epistropheus or axis. Our precise knowledge of the development of these two vertebrae rests principally upon the admirable researches of A. FrDric]), 83.1, 86.1, though previous investigators had establislieil that the first vertebni forms the so-calUxl (nlontoid process of the epistropheus, see Ch. Robin, 64.1, and C. Hassc*, 73.1. In birds, but not in mammals, the central ]K)rtion of the vorti^bral l)ow of the second cerv^ical segment also contrilmtes to the fonnati(»n of the epistropheus; in manuntds it disaj)j)ears or is merged in tlio intervertebral ligament. Owing to this <liffert»n<v» th'» atlanto-epistrophic articulation is not strictly homologous in the two class(>s, Ix'iiig formed in birds by the vertt»bral bow of the second segment; in mammals by the expanded (*uudal ])art. of the v<»rte})ral IxkIv of the first segment of the neck. The s|M'ciali/jition of the two vertelme l)egins when their chondrifi<'Htion is vv(»ll advaiu'ed (cow emlnyos, IT- is mm.), for we see then that the whole of the first vertebnil Ix^vv is changing into cartilage to f«)rm the atlas, and that it d<K»s not grow trjgether with the Inxly. M4»unwhih» in mammals the lj<Mly of the first vcTtobra is changing form, its (M»phalic end becoming conical to make the anlage of the odontoid process, and the caudal part broadening out, and making a shoulder laterally and ventrally around the base of the odontoid process ; this shoulder forms the articulation with the atlas. The expansion of the first vertebral body forces the vertebral artery and the second cervical nerve out laterally ; the bend of the artery thus produced is permanent ; the expansion also brings the first body into contact with the bases of the transverse processes of the second vertebra; the intervertebral tissue (ligament) between them disappearing; the two vertebrae unite by their two points of contact, and thereafter their fusion progresses toward the median line, until all the tissue of the intervertebral ligament is obliterated and the two cartilages have fused into one, the epistropheus.

The atlas ossifies from three centres, two of which correspond to and appear about the same time as those of the neighboring vertebral bows (neural arches), while the third does not appear until after birth, and is situated in the middle of the ventral arch of the atlas (corresponding to the primitive hj^pochordal brace, Froriep's Spange). Often there is also a separate centre for the spinous process. The two primitive centres unite on the dorsal side during the third year, and with the ventral centre in the fifth to sixth year.

The epistropheus, in accordance with its development, has four centres, one for the body of its first vertebra or the odontoid process, one for its own body, and two for its neural arches. The two first-named centres appear during the fourth or fifth month. The fusion of the centres may not be completed until the sixth or seventh year, and up to that age the tip of the odontoid process remains unossified.

Sacral Vertebrse

In man there are five vertebrae, characterized by their peculiar form and by their articulation with the pelvis, and which begin at eighteen years to slowly unite into a single bone known in anatomj^ as the os sacrum. In. other animals, however, the sacrum is not formed out of the same vertebrae, if we count from the last cervical vertebra, nor out of the same number of vertebrae. Various attempts have been made to explain these divergences — see especially Rosenberg, 76.1 — but no certain result has yet been reached. Of the historj' of these vertebrae we have no such exact knowledge as Froriep's researches have given us concerning the cervical vertebrae.

The processes form neural arches and lateral processes {Seitenfortsdtze) which were commonly homologized with the costal processes of other vertebrae, principally upon comparative-anatomical grounds. The chief embryological evidence in favor of this homology was the fact that the lateral processes have a separate centre of ossification, making, together with the three usual centres, five primarj' centres for each sacral vertebra. In 1S75 Rosenberg, 75.1, showe<l that the anlages of the sacral ribs c^m be seen in human embryos, and that the proximal ends of these change into Ciirtilage and fuse with the true transverse processes of the vertebrae — very much a.s happens with the thirteenth rib in man.

Coccygeal and Caudal Vertebrae

Behind the sacrum there are nine segments to be found in the human embryo of 8-0 mm., as discovered by H. Fol, 85.1. From the sacrum tailward they are found prc^ressivoly more and more rudimentary, and only from three to tive of the segments immediately following the sacrum developed ossified vertebne. These are the so-called ctxxrygeal vertebrae, concerning the embr>'olog>' of which we know nothing. It is probable that some of the segments behind the c*occyx form at least mesenchymal, if not cartilaginous, vertebne, and Fol's obser\'ations suggest that the last coccygeal vertebra is really the product of the fusion of sevenil caudal vertebne.

Only the first coccygeal vertebra begins to ossify before birth. This, the thirtieth vertebra, has been shown by E. Rosenberg, 76. 1, to be in the embryo really a sacral vertebra, but it sepanites in the course of development from the sacrum, and becomes the first of the coccygeal series.

=Ribs and Sternum

The ribs and sternum are vertebral structures, and theivfore strictly segmental. This statement seems to iiie an unavoidable dtnluction from Froriep's obstTvations on the development of the C(^stal processes of the vertebne, but it is directly opposed to the conception current among morphologists, according to which the ribs are interst^gmental. That the sternum is a inor
Ehological product of the ribs is, I believe, the accepted opinion of oth comparative anatomists and embryologists. That it is so in man has lx*on put lx?yond doubt by (f. Ruge's investigations, 80.1, set? also C. K. Hofmann, 80.1.

1. Ribs. — ComiMirative anatomy renders it probable that every vertebra had ribs primitively, and most of them have still in the human embryo the anlages of ribs. In man there are only twelve vertebrc'e (eighth to nineteenth) of which the costal anlages are represented in the adult by true ribs; traces of a thirteenth pair of ribs belonging to the twentieth vertebra ap|)ear in the human embryo, and as a rare anomaly the thirteenth pair occurs in the adult. In the cervical ivgion there are found costal processes of the vertebrae, also in the lumbar and Kicral region ; in the last-named region the processes accjuire a certain indejx^ndence, but s*xm lose it and fuse with the vertebne pro|)er. These variations should 1k» ])orn(» in mind wliile reading the following paragraph, which attempts to give the general history of a typical rib.

the ends of tf le vertebral 1k)ws grow out as shown by Froric]), 86. 1 , imtil they comi in contt^ict with the muscle plates of their own segments. By the bulging of the plate the end of tht» lx)W is forced to ex])and dorso-ventrally, and there is thus given the primary division intodorsjil or neural, and ventral or costal pnK^ess. The spinal ganglion forces the end of the bow, compare Fig. 24;5, j). 4:25, to grow toward the posterior limit of the segment, and this permits the costal ])rocess to grow out past the caudal edge of the muscle i)late and to then^ become the anlage of the rib, which is not therefore an interS(»gmental structure, as current tradition has it, but truly segmental ; the rib and the myotome headward of it l)elong to the same somite, and the rib owc»s its apparently intersegnipntal iH»sition to its situation at the caudal limit of the segment, Ix'hind tluMuuscular anlage. Whether the costal anlage is prinluctMl as an actual outgrowth of the condtniseil mesenchyma of the vertebral blastema or by differentiation of the mesenchyma /// loco, we do not know: nor do we know what limits the rib in the transverse plane so that it is merely a rod and not a wide and high partition wall. In this stage the rib is directly continuous with the vertebra, but when by changing into cartilage it passes into the next st£^, it separates from the vertebra by the development of a fibrous ligament, forming the primary articulation between the rib and spinal coliunn. The division takes place obliquely, thus allowing the head of the rib to come in contact with the body of the vertebra, and to articulate, by its dorsal surface, with the ventral surface of the future transverse process. In the course of its further development the single primitive articulation becomes divided and the secondary, or adult condition, is established with one articulation with the transverse process, and a second with the body of the vertebra. In the case of the ribs, which become rudimentary, the development ceases at this stage, and only the proximal end of the rib chondrifies; the small remnant of cartilage unites with the transverse process of the vertebra, re-establishing by a secondary union the primary connection.

The true ribs, as those belonging to the dorsal vertebrae of mammals are called, extend a considerable distance through the somatopleure toward the median ventral line, but, as discovered by H. Rathke, 38.2, 365, before they reach the middle ventral line the ribs produce the anlages of the sternum, and of the intercostal ligament, at first as condensed mesenchyma, which afterward becomes histologically differentiated — see the next section on the sternum. The ribs extend to unequal distances, the first coming nearest the ventral line, the last terminating farthest from it. In the human embryo of from 2 to 3 cm. there is present a thirteenth true rib (Rosenberg, 75.1, 89-91); the proximal end chondrifies and fuses with the vertebra. This valuable observation shows that the so-called fir^t lumbar vertebra of man is really the last dorsal vertebra, and in its embryonic stage is strictly comparable with the thirteenth dorsal vertebra of Troglodytes. As in Hylobates the twenty-first vertebra sometimes has ribs, the evidence within the primates suflices to prove that the lumbar region was evolved at the expense of the dorsal.

The ribs are only partly ossified, hence the osseous rib of the adult represents only a portion of the whole primitive rib, the most distal part of which has been reserved to contribute to the sternum (or intercostal ligament), and another part of which remains in the cartilaginous stage to unite the costal bone with the sternum or intercostal ligament. Each primitive rib is therefore divided into three parts: 1, the proximal bony division, the rib of human anatomists; 2, the middle cartilaginous division, the costal cartilage; 3, the distal sternal or ligamentous division. By the differentiation of fibrillar tissue out of the original costal anlage articulations are developed for the costal cartilages at their proximal ends with the bony ribs, and at their distal ends with the sternum. The exact history of these differentiations has still to be worked out.

The ossification of the ribs begins during the second month, according to KoUiker, and there is but a single centre. Schwegel, 58. 1, states that epiphyseal centres appear eight to fourteen years after birth in the head and tubercle, that is, for both vertebral articulation»; the epiphyses do not unite with the main bone until later; often not until the twenty-fifth year.

2. Sternum. — The breast bone is developed from the ends of the ribs, but the early stages have still to be ascertained by following out the relations while the anlages are in the mesenchymal stages. Hitherto investigations have beg^un only with the cartilaginous stage. It seems probable that the costal anlages grow beyond the ventral limits of the muscle plates and then bend headward, and by uniting, form a longitudinal sternal anlage on each side at some little distance from the median line. The cartilaginous half-sternum apears in rabbits the seventeenth day ; they are still separate in chicks of the eighth day, in piff embrj^os of about 27 mm. in human embryos of 24 mm. In the chick the halves are imiting during the seventh day, and in pig embryos of about 50 mm., the halves are fully united. The sternal anlages (Ruge's Sternnlleisten) arise from the ends of the first to seventh ribs, and accordingly are nearest together toward the head and diverge tailward. My own observations lead me to think it probable that the connection really extends to all the ribs, but between the seventh and twelfth ribs it becomes fibrillar, and gives rise to the intercostal ligament, which, therefore, is morphologically the prolongation of the sternum. The sternal halves gradually coalesce, beginning at their upper ends. In many mammals the sternum shows plainly its metameric origin and consists of separate pieces metamerically arranged, and there is a separate centre of ossification for each piece. In man, on the contrary, the originally continuous cartilage forms three pieces, the uppermost of which belongs only to the first sternal segment or first pair of ribs according to G. Ruge, 80. 1, but, according to W. K. Parker, also is formed partly at the expense of the aborted last cervical rib ; the middle piece corresponding to the second to seventh segment; and the third piece, which remains chiefly or wholly cartilaginous. The first piece is the manubrium^ the second piece is the body of the sternum, and the third piece is the ensijonn or xiphoid cartilage, G. Ruge, 80.1, found in human embryos two small suprasternal cartilages which fuse with one another and then with the manubrium ; the significance of these cartilages is uncertain.

The sternum ossifies with one centre in the manubrium, and in man with an irregular number of centres in the body. Its ossification does not begin until the sixth month.

The double origin of the sternum and its dependence upon the ribs was discovered by H. Rathke, 38.2, 3G3. This discover}^ was confirmed and extended twenty years later by W. K. Parker, 58.2, and more recently by A. Goette, Hofmann, 80.1, and G. Ruge, 80.1; the last is an admirable investigation of the development of the sternum in man.

TrabeculaB Cranii. — H. Rathke discovered that at the same time that the cartilaginous tissue develops in the occipital skeleton there appear two curved bars of cartilage in front of the not<x;hord. These cartilages by their fusion and expjmsion form the whole of the prsechordal chondrocranium, and were named by Rathke the trabecule cranii. All subsequent writers have made Rathke's discovery the starting-point of their accounts of the development of the anterior part of the skull. But the morpholf^cal differentiation of the Bkeleton, as we have already aeen in the case of the vertebne, ete. , is given by condensed meaenchyma, and the cartilage, when it firet appears and for a considerable period afterward, does not by any means correspond to the real shape of the skeletal piece. Now nearly all ^e information we possess as to the etirly stages of the skull is concerning the progress of the so-called chondrocranium, and since this is really for a considerable period merely the history of the progress of chondrihcation in the already formed mesenchymal skeleton of the cranium, it results that concerning the early stages of the skull we have almost no available information, nor can we hope to understand the morphology of the skull until its developmental history through the mesenchymal stages shall have been followed, as has been that of the cervical vertebrae by Froriep. Concerning the history of the cartilage of the skull we possess an immense fund of information, owing chiefly to tlio long series of splendid monographs by W. Kitchen Parker (18G6-1&86), the chief results of which have been summed up by himself and llr. Bettany in a single comprehensive volume, 77.1.

From what has been said it is clear that the shape of the pnechordal cartilaginous skull has verj" little morpholc^cal signiScance until the mesenchymal skull is completely chondrified; until then the growth of the cartilage represents merelythe advance of a histological modification within the skeletal piece. Unfortunately it is impossible at present to say when the cartilage does begin to represent the shape of the cranium. As the history of the early stages of the prae-chordal cartilage has very little morphological value it may be very briefly given.

The trabeculae cranii of the pig* may be taken ns typical representatives of the mammalian trabeculai, and show essentially the same arrangement as are / found in all other vertebrates, although I the form and proportions vary from ' class to class. In pig embr>-os of about Hi mm,, the trabecute cranii appear as two curving rods of cartilage, united in front, but separated behind ; in general 'shape they resemble calipers; ■they lie anteriorly between the olfactory pits Fra ta and the brain, and form from the start tS™ v Directory

either side of the palate in the mouth cavity behind the olfactory pits. These pits are situated entirely in frout of the trabeculsB at this stage, but between them there is an intemasal septum of mesenchyma, and into this Heptum there already extend two cartilaginous laminra which are the prolongations of the trabecuhe. In tho course of their further development the trabecule fuse throughout their entire extent. In pigs Op one inch long the intenmsal

■' cartilages have nearly or

quite fused into a single median piece, and the trabe* cuke proper are united also per except around the hypophysis, which they closely embrace. At this stage we see further that the trabecular cartilage is extending sideways, outward and upward arotuid the brain, outward and downward around the

„.^ ;ii^,?r rth°r.5'J3';'?S2 ^i oUactoo- pit>*. in embryos

"For'"^pi^^o^ti^'^i^'i^^.7' iV '^^^ ^?d "" third long the jxistenor ends of the trabeculse have united with the anterior end of tho occipital cartilage, thus forming a continuous floor of cartilage, which imderlies the brain, and in front overlies the olfactory pits, and has also odevelped under the hypophysis, which thus becomes definitely separated from tho mouth ca\ity and inclose<l within the brain case. We find at this stage also that the cartilaginous pcriotic capsules have begun to fuse with the lateral portions of tho occipital cartilage, thus making one continuous skeletal piece, which is known as the primitive chondrocrauium, but it does not correspond to the real cranium at this stage, for beyond the limits of the cartilage the skeleton around the brain anil olfactory pits is already formed as condensed mesenchyma. The general arrangement and the outgrowths from tho trabecular mass are shown in Fig. 248. The hypojjhysis, Hy, lies in a deep fossa, which remains in the adult and i« known as the sella turcicit; on the caudal side of the hypophysis the fused ends of the tral)ecul£o have risen as a transverse plate, the jxisterior clinoid ridge, p.cl, and in front of tlio hypophysis is the much smaller anterior clinoid ridge ; p.sp indicates the region of the future" pr»-aphenoid bone, the cartilage of whicli is wntinued directly forward in the nasal septum as the ethmoidal plate; from the sides of cartilage there spring two lateral plates, which our\"e upward and outward around the brain ; the anterior and Lii^r of these plates is the orbito-sphenoid, O.ap, which spreads ont between the brain and the eyeball, and extends far hack toward the i)erif.>ti<! capsule, per ; during its development the orbito-sphenoid cartilage grows around the optic nerve, thus forming the optic foramen. Op, which is near the base of the plate: the smaller of the plates is the ali-s))Iienoid, aud springs from the r^ou of the two clinoid i-idges: it is short and thick and has a downweu^ process, which extends to the palatopterygoid bar and represents the external pterygoid cartilage; this process being external does not show in the figure. Between the ali-8phenoid and the periotic capsule is a shallow foeea for the Gasserian ganglion, and from the ganglion the main stem of the fifth or trigeminal nerve passes out through a foramen. The superior maxillary division of the trigeminal passes out between the orbito- and ali -sphenoids. The nasal cavities are large and complex ; they already occupy more than half the length of the head, and in part underlie the brain; the partition which separates the nasal cavity from the overlying olfactory lobes is composed of undifferentiated mesenchyma, which is traversed by the olfactory nerve fibres, but at the present stage, or a little later, the partition chondrifies by an extension of the cartilage of the ethmoidal plate, with the result of producing the cribriform plate, cr. p. The shape of the nasal chambers is rendered complex by the turbinal prominences on the lateral wail of each chamber as described in Chapter XXVIII. Already in the previous stage the median ethmoidal plate had sent outgrowing lamiuEB of cartilage one on each side over the top and down on the outside of each nasal cavity, and from the lateral cartilage there appear ingrowths into each turbinal prominence. The relations of the cartilage to the nasal chambers can be more readily

understood in a cross-section. Fig 249, which calls for no further description than is afforded above and in the explanation of the figure. As partly indicated by Fig. 248, there are five turbinal prominences, the ali-nasal, the inferior, tt, the middle, nt, and the upper «. tb — the last two mentioned being, however, hardly distinct from one another at this stage. It now remains only to add that at the ventral side of the anterior eilge of the ethmoidal plate the cornua trabecularuni are still present ; the cornua are the anlages of the ali -nasal cartilages.

In man the history of the chondrocranium is very similar to that just given for the pig, as we know through the investigations of Spond&i, 46.1, Vrolik, 73.1, Virchow, 57.1, and Van Noorden, 87.1, and others.

The general significance of the chondrocranium is discussed in the section on the morpholog}' of the skull, p. 405.

Concerning the origin of the ixjriotic capsules we possess no accurate knowledge, and cannot even say whether they represent primarily distinct skeletal i)i(M'(;s or mc^vly separate centres of chondrificatiou in a larger nu»sonchymal skeU^tal piece. The latter appears to me the more probable alternative, ami it may be further suggested that the Ciii)sules are diffen^ntiations of the lateral outgrowths of the investing mass of the cephalic^ notochord. The ({uestions raised can be answered only by a careful investigation of the mesenchymal cranium.

Ultimate History of the Chondrocranium. — The primitive cartilaginous skull is fornicMl by the fusion and expansion of the occipital cartilage, the tralj<Hniho cranii, and the ix?riotic capsules. It occupies the floor of the (Tanial cavity and the vooi of the olfactory cavities, and Iwis certain latijral expansions. The arrangement of these can be underst<j<Ml from the acco]nj)auying Fig. '^oO, although the figure represents a sUige in which ossification has l>egun. Between the nasal cavities lies the mesethinoid septum from the dorsal side of which spring the ali-njusals, o//i, covering the dorsal and lateral parts of the nasal (?aviti<is; from the mc^sethmoid extend also the plates forming the ali-ethmoids and middle turbinal, mth ;

also tho cribriform plate, cr, through which the olfactorj- nerve

passes. The orbito-sphenoidal wings, obs, are large and rise from

the prse- sphenoid ; the ali-sphenoidal wiugs are smaller, al ; between

the tvro sphenoid wings is the foramen lacerum; the peiiotic

capsules aro large and fill out nearly the

whole space between the ali-spheuoids and

the wings of the occipital. the occipital has

expanded completely around the foramen

magnum, /.fH, through which the spinal cord

enters tlio brain-case, so as to form on the

dorsal side tho supra-occipital, s.oc.

In the fishes the chondrocranium passes through a' stage corresponding closely to that just described, except that in them there is ^

no Imno formed; but whereas in the mammal ,' ^ '^J' '

tho chondroci-anium does not pass beyond ■^

this stage, iu the fishes it continues growing " ^

until tho brain is completely inclosed and

there is a jierfect cartihiginous skull, at least ^ •*

in tho lower forms, maraipobranchs, ganoids, \

and selachians. We must, then, distinguish a ^ r

two tyiws of chondrocranium, according as ^«^ '^-^ ^ it does or does not completely encase the j '""^i.^ -^oo brain. The latter is the type exclusively ^^/^ _EDboo pir -i, found m ntammalia. fS!^™ '■'™^' ^*"'y os»in«i

The manuiialian chondrocranium is repre- ab^v^^'^" ai™ ai-^T^h.'S" sented in the adult by a number of distinct ^■ci?[i,r!(^^ SiJlte'-liJ-T.ririto^ bones, which represent also a still larger Bphii»)id:(ij,aii-Bi>in'iioid:j*i-, number of bones of lower types. As to how Kli^Vf /BrV.'iTOrnpn'maS^iiSl the originally continuous cartilage becomes ^^' *A!r'ter'w '^"pamer"' divided into separate bones, our notions are

somewhat vague. In tho division the centres of ossification play a leatling role, of course, but not in the sense that every centre invariably results in the formation of a separate bone. The second importimt factor is the development of the sutures, which form the boundaries of the l>ones. The sutures are of two kinds, those marked out by tho edges of the cliondrocranium itself, and those produced in the cartilage. Although a knowleiige of the history of the sutures must be considered of the utmost importance for the elucidation of the morpb' ■■ "^' of the skull, such knowledge appears never to have been sought. ^ides those parts of the cartilaginous skull which make bones there are certain others, few in number and small in size, which atrophy. We have then to present the hi8tor3-of the ossification and partial atrophy of the chondrocranium.

Ossification'. — The occipital reqion begins to ossify during the early part of the third month in human embryos; comparative anatomy teaches that the occipital bone of man is homologoiis with five bones — the median ventral basi -occipital bordering the front or ventral side of the foramen magnum, the paired lateral exoccijiitals bordering the sides of the forameu and including the iiindyles by which the occiput artitiilates with the axis, and the paired supra-occi pi tals, which, however, are often united into a dorsal median bone; in agreement with this homology there are five centres in the occipital cranium, namely, the basi-occipital, the two ex-occipital or condylar, and two supra-occipital, which, however, very soon unite; according to KoUiker there is also later a small deposit of dermal bone added to the supra-occipital. The ex-occipitals do not unite with the supraoccipitals until one or two years after birth, nor with the basi-occipitals until the fifth or sixth year. In the sphenoid region ossification begins during the second half of the third month in the human embryo, and takes place from six principal centres corresponding to the six bones with which the human sphenoid bone is homologized by comparative anatomists. The six centres are: 1, the basi-sphenoid in the neighborhood of the hypophj'sis, and said by Kolliker to be due to the fusion of two minor centres ; 2, the pre-sphenoid, which appears in the median lino ne^ir the optic foramina, and is likewise said to consist of two minor fused centres; the pre-sphenoid, at least in the pig, is the last of the six centres to ap^K^ar; !5, 4, the ali-sphenoid centres, one in each wing, Fig. 250, al : they appear a little later than the basi-sphenoid centre ; 5, (J, the orbito-sphenoid centres, which unite with the prae-sphenoid after the fifth month ; the praesphenoid and basi-sphenoid do not unite until several years after birth, and even at thirteen years Virchow has found remnants of cartilage between the two bones. In the periotic region there are three main centres, which are taken to represt»nt as many distinct bones, although they unite in mammals into a single bone, the os petrosiim; in man the petrous bone is found to have fused with the dermal bone, known as the squamosum^ and also with the ring of bone fonnecl around the tympanum of the ear, and known as the annulus tynipanicus; from the union of these five bonos arises the temporal bone of human anatomy. The three centres which ap[>ear in the periotic capsules are termerl the pro-otic, opisthotic, and epiotic, and are considered to represent the separate Ixjnes bearing the same names in lower vertebral :s; the pro-otic centre is by its ix>sition in close relation with the anterior vertical semicircular canal, between which and the exit of the third division of the fifth nerve it lies ; in pig embryos of six mches it forms a patch of bone lying under the fore part of the cochlea alx^ve and in front of the fenestra ovalis, and extending to the junction of the anterior and i>osterior semicircular canals; the oi>isthotic centre is on the lower and posterior surface of the cai>sule, placinl so that most of the bulbous ix)rtion of the cochlea lies dorsid to it; one of its processf^s lies between the fenestra ovalis and the fenestra rotunda, close in f rt^nt of the head of the stylo-ln'al cartilage; the epiotic centre develops somewliat more tardily; it is in es|HH'ial relation with the posterior vi^rtical semicircular canal, and when it first apiH\'irs (pig embryos of six inches) is a small piece just alnn'e the stylo-hyal pi\x»ess and foramen rotundum, and behind lx>th the foramen ovale and the alx'>ve-mentioniHl opisthotic process. Acci^nliugto A. J. Vn>lik, 73.1, the ossification of the jx^riotic capsules pnxH^^ls somewhat diffen^ntly in man, there Ix^ng four centres which iHvih^iv bv the sixth month of fa^tal life. In the rthmoichil mjiofi^ inchuling the cribrifonu plati\ the lateral nasjd and turbinal cartilages, ossification takes ])lace very late, and the mor])hological significancH? or liomologies of the various tvntn^s is little understoixl.

In the pig at birth the median cartilage is unossified, the cribriform plate is about to begin ossification, being invaded by vascular mesenchyma, the upper and middle turbinals are partially ossified, the inferior turbinals almost completely ossified. In man a similar condition is reached about the seventh month of foetal life. The human ethmoid proper does not ossify until the first year after birth.

Atrophy. — There are certain parts of the chondrocranium which do not ossify, but are lost in the adult. The exact process by which they are resorbed is not known. The following parts are said to disappear: the comua trabecule; 2, the cartilage under the nasals; 3, Spondli's so-called frontal plate, or that portion of the orbito-sphenoid outside of which the frontal bone is developed ; 4, the parietal plate or a small portion of the ex-occipital outside of which the parietal bone is developed ; 5, a small portion of the ali-sphenoid (ala magna) outside of which the parietal bone is developed ; 6, the cartilaginous capsules of the sphenoidal, maxillary, and frontal sinuses ; ?, parts of the turbinal cartilages.

Dursy, 69. 1, 203, has maintained that some of these cartilages do not really disappear by atrophy, but by becoming ossified and united with the dermal bones overlying them. KoUiker (" Entwickelungsgeschichte," 456), without absolutely denying the correctness of Dursy 's view, states that he has been unable to confirm it by his own observations.

Branchial Skeleton. — Every branchial arch contains a skeletal element, which in its primitive form in all vertebrate embryos * is a bar or rod of condensed mesenchyma, which very early changes into cartilage. The number of these bars of course depends upon the number of gill-arches, compare p. 203, and hence in the mammalia there are five branchial cartilages on each side, which begin dorsally near the cranium, and curv^ing around the sides of the pharynx end near the median ventral line. Fig. 177. The position of the cartilage can be seen in a section of a branchial arch. Fig. 152, to be alongside of the arter}^ or aortic arch, and on the pharjTigeal side of the coelom of the branchial arch. The constant recurrence of the simple stage just described in all vertebrates (except, perhaps, in marsipobranchs), renders it highly probable that forms existed at oi\e time with such a branchial skeleton; but no such forms are kno\vn to exist at the present day.

It will be convenient to stiite the divisions which comparative anatomy teaches us may be considered typical for each branchial cartilage. The divisions are usually given as four: 1, pharyngobranchial, or dorsal segments, which has usually a horizontal course; 2, the epi-branchial, and, 3, cerato-branchial, both at the sides of the pharynx; 4, the hypo-branchial or ventral segment, which typically articulates with a me<lian unpaired cartilage known as the basibranchial, or copula. In the acpatic vertebrates the bars usually send out supporting cartilages into the branchial lamelke, but in mammals there is no trace of any similar outgrowths even during embryonic periods.

Except perhaps, in the marfiipobranchs, the branchial skoh'ton of which is i)«)ssil)ly not homologous with tbat of the higher vertebrates. See, however, Auton Dohm, 84. 1.

In mammals the earliest stage of the branchial skeleton has never been accurately described; this is because investigators have hitherto been content to begin with the cartilaginous stage, instead of the mesenchymal stage, and, consequently we are left with no definite information as to the bars of the fourth and fifth arches, and with insufficient information as to the origin of the bars of the first to third arches. In selachians, according to Anton Dohm, 84.1, 110111, the differentiation of the cartilage of the branchial arches begins shortly after the branchial filaments have appeared as a condensation of the mesenchyma. Fig. 152, (7, situated on the pharyngeal side of the arch and tailward of the mesothelial anlage, In.m, of the inner muscles. For the further history see Dohm, /. c, 114. In regard to the history of the branchial skeleton from the cartilaginous stage on, we have very full information, chiefly owing to the extensive investigations of W. K. Parker, also in part through Kolliker, DoUo, Salensky, 80. 1, Fraser, 82. 1, and others. Each pair of bars passes through a distinct series of modifications, therefore it will be convenient to present the history of each pair separately. We shall call the skeleton of the first arch the mandibular bars, that of the second the hyoid bars, of the third the thyro-hyal bars.

Mandibular Bars. — The adaptations of l)oth the mandibular and hyoid bars to functions entirely different from those which they primitively served, are most remarkable. In mammals the mandibular bar becomes primarily divided into two parts, a dorsal piece corres|X)nding to the palato(iuadrate of comparative anatomy, and a ventral piece known as Meckel's cartilage. The commencement of the corresponding division of the mandibular bar may be seen in a dog-fish embryo of about 23 mm., the upper end of the bar being enlarged and sending out a process which runs forward on the cranial side of the mouth and later joins the tralx»cula ; this process is the palato-pterj'goid ; another process, the meta- pterygoid, runs upward; the wider part uniting the two processes is homologous with the quadrate; in elasmobranchs the meta-pterygoid process becomes ligamentous. In mammals the early stiiges have not been worked out. Parker states that in embryo pigs of about 16 mm. the cartilaginous palato-pterygoid bars. Fig. 247, are less definitely developed than the other skeletal elements present at this stage, but are more or less distinct from the rest of the mandibular l)ar ; the palato-pterygoids are situated in the maxillary process, so that, starting from the dorsal end of the mandibular arches, they run obliquely downward and forward toward the anterior end of the tralx^culre ; anteriorly, they converge toward the median line, but do not meet. In the mandibular arch itself is the rod-like Meckel's cartilage. Fig. 247, Md. Between the pterygoid plato and the cartilage of Meckel is a space in which Parker figures no skeletal element, but which is occupied by the quadrate element, which in mammals is the anlage of incus. At the same stage (embryo pig, 10 mm.) the lower divisions of mandibular bar or the Meckel's cartilages are much stouter and are better differentiated from the mesenchyma than the palato-pterygoids; they are situated in the mandibular processes, and do not meet in the median line. Each Meckel's cartilage is a rounded rod, but its dorsal extremity forms a hook, is somewhat enlarged, and is situated close to the upper border o' the first branchial cleft. In pigs a little older (25 mm.) the hook is longer and the end of the cartilage is thicker, making it easy to recognize in it the anhigc of the malleus, the hook being the future manubrium or handle of the malleus. In pigs two and one-half inches long the malleus is bciwirately ossified, but is not separated from the cartilage of the jaw. When the final separation takes place I do not know.

Meckel's cartilage proper may be defined as the ventral segment of the first branchial bar. In mammals the two cartilages always unite in the median line, although in man the actual union is said not to have been observed. The lower portions of the cartilage oHnify metaplastically but not to the median line, and this ossification l>egins in man during the third month. The bony part is incoriKjratod in the permanent mandible, but the rest of the cartilage atrophies and entirely disappears except a small portion of the end next the malleus, which becomes changed into fibrillar tissue and remains, according to KoUiker, "Gnmdriss," 320, as the ligamentum late rale internum maxillae inferioris. Meckel's cartilage is the homologue of the cartilaginous mandible of the lower fishes, but is not homologous with the bony mandible of the amniota, which is devel()i>ed later and belongs to the class of the dermal bones.

Summary, — The primitive cartilaginous rcxl of the first branchial arch gives rise first to a palato-quadrate dorsal segment and a ventral or Meckelian segment. The palat<j-quadrate segment sulxli vi<les into the palato-pter>'goid plate and the quadrate or incus. In the earliest accurately known mammalian stage the palatopterygoid and incus are already separate, but it may be safely- assumed that in a still earlier stage they constitute one piece. The Me<;kelian segment subdivides into the malleus and the Meckelian cartilage profK»r; the latter unites in the median ventral line with its fellow. One inevitably inclines to homologize the parts with a typical }>ranchial arch as follows: The palato-pterygoid is the phar>'ngo-}>raiichial ; the incus is the epi-branchial ; the malleu.s is the ceratf>branchial , the Meckel's cartilage is the hypo- branchial; the united ends of the cartilages are the copula. These homologies are, howev<»r, somewhat hyj)othetical, principally l)ecause the homologi(?s of tho malleus are not clearly a.scertained, and we cannot say what element of the lower vertebrates it represents.

The course of the palato-pter>'goid at such a markwl angle to the Meckel's cartilage is probably due to the head-l)end. Verj' likely the head-bend is causally connecterl aLs^> with th<; ixxruliar fonns assumed by the incus and malleus.

Hyoid Bars, or Reichert's cartilages, as they have lif^-n namwl by KoUiker, are the skeletal elements of the wxrond or hyoid branchial arch, and thev are t^'picallv di\nded, like the other bars in the lower vertebrates, into four parts, the dorsid one <jf which (pharyngobranchial) fu.ses quite early with the cartilaginous jx-riotic capsulf.»H, and becoming ossifie<l appears in the human adult as the nfifloid process; the second part (epi-branchial) l^ecom^'s f>?irtly ligam^Titous in all placental mammals, and perhaps whr»lly ligamentous in man; the third part (cerato-branchial) and fourth part (hyjK>-branchial) both become cartilaginous and ossify early, so as to form a single piece of bone, which perhaps includes also some bone derived from the second part also. This single piece of bone is known in the adult as the lesser horn of the hyoid. The adult hyoid bar then comprises the styloid process, the stylo-hyal ligament, and the lesser hyoid comua. The main body of the hyoid probably belongs to the next branchial arch, but the hyoid bars unite with it very early.

It was long maintained by Huxley, 69,1, and W. K. Parker (Parker and Bettany, " Skull") that the incus was derived from the hyoid bar, but since Salensky, 80, 1, showed that the incus is developed from the mandibular bar, Parker, 86.1, 10, has retracted his former opinion. Reickert, 37.1, thought that the stapes was derived from the hyoid bar, but recent investigations show that this is not the case, although Rabl, 87. 1, has shown that the stapes is developed within the territory of the second branchial arch. O. Hertwig ('*Lehrbuch," 3te Aufl., 500) suggested that the stapes was a double bone, one part of which is derived from the branchial skeleton, but Staderini, 91.1, has proved that this suggestion cannot be adopted — see Chapter XXVIII.

The following quotation from W. K. Parker, 86.1, 10, 11, gives some insight into the discussion about the incus, which may be said to have ended witji the admissions made in the course of the quoted sentences. " But that great improvement just spoken of as appearing in the organ of hearing in the mammal has wrought a change in the hinder face that has two most important bearings. From the first promise of an ear-drum in the tailed Amphibia, to its highest fulfilment in the noblest of the oviparous tribes — the birds that nestle on high ('aves altrices') — the only element from the visceral arches that is used for carrying the vibrations of the air inward to the organ of h€»aring is the uppennost part of the hyoid arch — the

pharyngo-branchial ' element of the second postural arch, to speak morphologically. From the salamandroids to the singing l)irds, all through the Amphibia and Sauropsida, the first jvjstoral arch which forms lK)th the upj>er and lower jaw is only sogmontod once, that is, into an epi-branchial and a corato- branchial eloment or joint. The upper piece is specially termed the 'quadrate ' and the lower the *articulo Meckeliaii ;' the one forms the swinging piece, hinge, or pier, to the ' com])i)un(l lower jaw,' and the other its axis or pith, the

J)art which becomes coviTcd with more or fewer "investing Ixmes.' n these low ' Eutheria ' and also in lx)th the ' Metatheria ' and the

Prototheria ' (Marsupials and Monotremes), the modified visceral rod that runs tlirough the drum cavity has two new elements added to the one (single or variously segmented) element derived from the hyoid arch. This is an apparently snchlen chaujj^e, for we have it in the low(»st or teatless mammals ; their f///fr.s7/7/that should show us the earlier steps of the change are unfortunately all extinct. In this dilemma not only zo«)logy, but palaeontology also, fails us utterly, but embrj-ologj' comes in with every stage and every link. I have workeil out the early conditions of these j)arts in sev(?ral kinds of Marsupials, and in the young of 0^nitll()rh>^lchus; Init even in the lower Euthreia, the Edentata, now to \ye described, and in the large and varied group of the Insectivora, I have been able to trace ever>' step in the transformation of these parts. I am now satisfied that the incus is the upper element of the first or mandibular arch ; both Professor Salensky's and Professor Fraser's researches put this, I think, beyond doubt; and my own attempts for a long time to make the hyoid theory of this part agree with facts, only kept the subject in hopeless confusion. The new elements of the ear-chain are then the arrested quadrate or incus, and the arrested and amputated articular region of the articulo-Meckelian rod or primary lower jaw. The bony part of the ' ramus ' is the well-known dentary with the coronoid and splenial bones in a sub-distinct state ; the cartilage for the new articulation ot the lower jaw is derived from a large superficial slab — a ' lower labial ' — the like of which is not found again until we get as low down as the Chimseroids. From this is derived the hinder half of the ramus by transformation of its substance into bone ; and from this we get the cartilage, both of the condyle and the glenoid cavity, and also of the intervening ' meniscus. ' Of course the drum cavity is the * first cleft,' and the concha auris with its segmented meatus-tube — the tyn^panic bone, the tympanic bulla, and the cartilaginous lining of the Eustachian tube — all these are parts of a curiously specialized opercular growth belonging to the hinder edge of the first visceral fold and arch. This last assertion has not been made as a stride across the types from the mammal to the elasmobranch, but is the result of a very slow step-by-step process, made during many years ' along all the lines' of vertebrate morphology."

Thyro-hyoid Bars. — Whether these bars extend in the mesenchjTnal stage through the entire length of the third branchial arches or not is not known, but their lower ends are chondrified and later ossified to form the principal part of the hyoid bone. There appears very early a median azygous element or copula, which in pigs of IG mm. is already cartilaginous and unite<l with not only the thyrohyoid bars but also with the recurved ends of the hyoid bars. This copula is called the basi-hyal, and is the anlage of the main body of the hyoid bone; it is said to belong to the third branchial arch, although the hyoid bars unite with it. It is at first small in size, but as development progresses it enlarges considerably, while the ventral ends of the hyoid bars grow but little ; it results that the relative size of the parts is changed, and the rudiments of the hyoid bars, which start nearly equal in diameter to the basi-hyal, appear in the adult as the lesser horns. The thyro-hyoid cartilages, on the other hand, grow at about the same rate as the basi-hyal and becimie the greater horns of the adult hyoid bone.

The hyoid bone of mammals is formed by the ventral ])ortions of the hyoid bars (lesser comua) , the ventral portions of the thyro-hyoid bars and the copula of the third pair of bnmchial arches. In accordance with its development the hj^oid bone has five centres of ossification, one for the body and one for each of its four horns. Ossification begins in man in the great cornua and body during the last month of foetal life, and in the small comua during the first year after birth. The great comua and body do not unite until middle life, and the lesser comua usually remain distinct, though sometimes found united with the body at advanced ages.

II. The Limbs and Appendicular Skeleton

Origin of Vertebrate Limbs. — The morphological value of the limbs of vertebrates has long been the subject of discussion and speculation, and at the present time the solution of the problem is tneoretical rather than positive. It is unnecessary to give a resume of the older hypotheses as to the archtype of the limbs, though I may refer those interested to Owen's article " On the Nature of Limbs," and Goodsir's essay " On the Morphological Constitution of Limbs," Edinburgh, New Philos. Jbtim., 1857. Gegenbaur has advanced an hypothesis of the origin of limbs in support of which his memoir, 76.1, brought very scanty evidence. According to this hypothesis the limbs are modified branchial skeletons, the shoulder and pelvic girdles representing the branchial bar, and the skeletal pieces of the limbs proper representing branchial rays ; the central ray formed the axis of the limb, and the remaining rays gradually became articulated with the axial ray, and thus produced the type of limb found in Ceratodus, and which Gegenbaur regards as the primitive type from which all vertebrate limbs are derived. This theory, which was adopted by Huxley (on Ceratodus, Proc. Zool. Soc, London, 187G), has attracted great attention, although it has been definitely set aside by the observations of Balfour, 81.1, on the development of the limbs of Scyllium, which demonstrated that the liml^ arise as parts of a longitudinal fold, which runs along the side of body, both fore and hind limb being part of the same fold. Were Gegenbaur's hypothesis correct, the limbs should arise as transverse or vertical folds. Under these circumstances it seems to me that Gegenbaur's theory has merely historical interest.

The only theory having any standing at present is the one adopted by Balfour (" Comp. Embryology," II.) according to which the limbs are specialized portions of a lateral fin-fold, similar to the dorsal and ventral median fin-folds of fishes. The resemblance of the lateral fins or true limbs to the median fins in general structure is obvious in many fishes, and especially in teleosts, and renders direct comparison very natural. Such comparison is suggested by several writera, but was first definitely worked out by J. K. Thacker, 77.1, and at about the same time advocated by St. George Mivart, 79.1, both these authors basing their conclusions upon comparative anatomical studies. Their general result was that the structure of limbs could be explained by assuming that they are specialized portions of lateral fin-folds, having a structure similar to that of the median fin-folds. At about the same time appeared the chapter of Balfour's monograph on the development of elasmobranch fishes, in which he advocated a similar theory upon embryological grounds, and by his observations put the theory upon a firm basis. It is a remarkable coincidence that the same hypothesis was formulated independently and published at about the same time by three investigators. These views were attacked by Von Davidoff, 79.1, then a pupil of Gegenbaur's, and to Davidoflf's paper Gegenbaur added a note upholding his theory; these criticisms were adequately answered by Balfour, 81.1 ("Reprinted Works," I., 714).

From the manner of their development it is obvious that the limbs have a flattened form and a dorsal (or extensor) surface, and a ventral (or flexor) surface, and as soon as they project from the body, as they do at right angles, there is an anterior or cranial border and a posterior or caudal bJirder. The development of the limbs in Scylliura, as described by Balfour, throws important light on the primitive position of these borders. Balfour ("Comp. Embryol.," II., 612) says : " The direction of the original ridge which connects the two flns of each side is nearly, though not quite, longitudinal, sloping somewhat obliquely downward. It thus comes about that the at^chment of each pair of limbs is somewhat on a slant, and that the pelvic pair nearly meet each other in the median ventral line a little way behind the anus. The elongated ridge, forming the rudiment of each fin, gradually projects more and more, and so becomes broader in proportion to its length, but at the same time its actual attachment to the side of tho body becomes shortened from behind forward, so that what was originally the attached border becomes in part converted into the posterior border. This process is much more completely carried out in the case of the pectoral fins than in that of the pelvic, and the changes of form undergone by the pectoral fin in its development may be gathered from my figures. In Scj'llium the development of both the pectoral and pelvic fins is very similar. In both fins the skeleton in its earliest stage consists of a bar springing from the posterior side of the pectoral or pelvic girder, and running backward parallel to the long axis of the body. The outer side of this bar is continued into a plate which extends into the fin, and which becomes veryearly segmented into a series of parallel rays at right an- itJUf i?lo

fles to the longitudinal bar. 5!,'J^'j„™ ^ n other words, the primitive m^ta-pi*ryi

running along the base of the fin and giving off at right angles a series of rays which pass into the fin. The longitudinal bar, whieli may be called the basi- pterygium, is, moreover, continuous in front with the pectoral or pelvic girdle as the case may be. My observations snow that the embryonic skeleton of the paired fin consists of a series of parallel rays similar to those of the unpaired fins. These rays support the soft part of the fin, which ha-s the form of a longitudinal ridge, and are continuous at their base with a longitudinal bar, which may very probably l>e due to secondary development. As pointed out by Xlivart, a longitudinal bar is also occasionally formed to support the cartilaginous rays of unpaired fins."

Balfour's observations show that there was a primitive longitudinal skeletal piece at the base of the limb-fold, and that from this rays are developed which run out into the fold; ilivart assumed that tho rays were primitive and the longitudinal piece the product of the fusion of the bases of the rays. As the limb grows out its base becomes free and constitutes the posterior border, and the basal skeletal pioc*^ appears as the axis of the limb, while the tin-rays run off from one side toward the primitive outer or ultimate cephalic border of the fin; on tho caudal side of the axis there are neces&arilly no fin-niys. If we assume, as we must, that Scyllium illustrates the general type of fin development, then a condition in which, as in the fins of the adult Ceratodus, there are rays on both sides of the axis must be considered a secondary condition. The Ceratodus type is known as the archipterygium, and, as already stated, has been held by Gegonbaur to be the ancestral form of vertebrate limbs. But our knowledge of the development and morphology" of fins renders it impossible to accept this view, at least at present.

The archipterygium may be defined as a skeletal limb axis with rays coming off on both sides; no such fins are kno\NTi among the lower fishes, but only among the higher (Dipnoi) ; this fact offers another serious obstacle to regarding the archipteiygium as the primitive ancestral form, but suggests that it may represent the ancestral form of the ])entadactyle limbs of amphibia and mammals. I think much may bo said in favor of this suggestion, and indeed it is largely on account of the possibility of deducing the ])entadactyle limbs from it that the archipterj'gium has bc^en regarded i\s an archtyj>e by Gegenbaur and his followers.

The cheiropferyginm is the archtype or ancestral form of the pentadactyle limb. Its essential characteristic is the division into tour segments :

, S Upix»r ami. « j Foreami. « i Caqius. . \ IIau<l.

^ ] Ui)por h^i^. ^' / Ix)wer leg. ^' I Tarsus. "* '( Ft)Ot.

The upper segment contains one long lx)ne (humerus or femur) ; the second segment contains two long bones (radius or tibia, and ulna or fibula) ; the third segment (contains nine small bones (carpals or tarsals) ; the fourth segment consists of sep^irate digits, five in number, hence the term pentadactyle applied to this type of limb; each digit has a proximal or basal bone (metacarpal or met^itarsal) upon which follows a linear series of phalanges, separate bones variable in mmiber. It is convenient always to count the digits in the same way, commencing from the radial or tibial side; thus the thumb is the first digit of the hand, the great toe the first digit of the foiot.

The arrangement of the carpal and tarsal bones is greatly modified not only in the ajnniota but also in many of the amphibia, both by the suppression of some of the nine bones and by fusions among them. The nine bones are the intermedium betweini the distal ends of the radius and ulna, the radiale nnd uluare at the distal ends of the radius and ulna resi)ectively ; the two centralia, on the distill side of the ntermedium ; Ix^tween these four and the mebicarpals (or metatiirsals) follow the five vctrpalid or tarsal ia,. In most pentadactyle limbs the two centralia are fused into one tone, the cent rale. In many (*ases some of the bones are suppressed. The following table shows the homologies in man :

Ulnare* (fibulare) . Cuueiforme. Calcaneum.

Intermedium. Lunare. i Astrjiiroi„R

Radiale (tibiale). Scaphoid. ^ Astragaius.

Centralia. ( ?) Naviculare.

1. 1. Carj^ale. 1. Tarsale.

2. (Carpal ia). 2. 2.

3. (Tarsalia). 3. 3.

t' y Unciforrae. [ Cuboides.

The pisiforme is a sesamoid bone developed in the tendon of the flexor carpi ulnaris, and has nothing to do with the primitive carpus.

It is generally taught that there is one series of bones which represents the true axis of the limb, and that the other bones represent a series of rays coming off from it. This supposed axis begins with the humerus (femur), is continued through the ulna (fibula), and terminates with one of the digits, but which digit authorities are not agreed ; thus Gegenbaur carries the axis through the ulnare fifth metacarpal and fifth digit, which makes the first ray pass off from the humerus and include the radius, radiale, first carpal, and first digit; the second ray arises from the ulna and includes the intermedium, one centrale, and the second digit ; the third ray springs from the ulnare and includes one centrale and the third digit ; the fourth ray springs from the fifth carpale and includes the fourth carpale and the fourth digit; similarly, changing the names, in the hind limb, see Gegenbaur, "Grundriss d. vergl. Anatomic," 1878, 512, Fig. 2T3. Wiedersheim, on the contrary, carries the axis (see his " Grundriss der vergl. Anatomic, 2te Aufl., Fig. 110) through the ulna (fibula), intermedium, both centralia, second carpale (tarsale) , and second digit. Such divergences of opinion raise doubts as to the existence of any true axis at all.

A full discussion of the morphology of the limbs does not fall within the scope of this work, because our concei)tions are not based upon embryological observ^ations. I shall, therefore, merely refer to the recent papers of G. B. Howes, 87.1, J. A. Ryder, 87.1, D'Arcy Thompson, 86.1, Hatschek, 89.1, and E. E. Prince, 90.1.

Relation to the Somites

Each limb arises along the territory of several somites, and receives outgrowths from the muscle plates of several successive segments, and with these outgrowths, which produce the muscles of the limbs, come the nerves of several segments, so that the fact that the limb arises along a considerable length of the body explains several important features in the developmi'nt of limbs — features which remain inexplicable if we accept Gegenbaur's theor>' of the evolution of the limb from a branchial arch, because this theory- confines the primitive limb to a single segment, whereas at its very earliest stage it is already related to several segments. As to the exact number of limb somites we are in doubt. Balfour's observations indicate that each limb was originally attached fdong a considerable number of segments, but that on the caudal side the attachment becomes shortened. As it is not until this restriction of the hase has taken place, thnt the nmscle j)lates penetrate the limb, it follows that the nmscles of the limb are derived from a less numl^er of segments than corresponded to the primitive attachment.

This reduced number is probably five in the amniota, but certainty on this point is yet to be reached.

Concerning the position of the limbs, as regards their distance from the head and the segments to whicli they belong, we have little exact knowledge. A. M. Paterson, 91.2, holds that the position in this sense is not uniform among the mammalia ; he bases this opinion upon the innervation which is variable. The variation is much less for the fore than the hind limb; the former is, as a rule, innervated from the lower cervical and upper thoracic segments; the twenty-fifth spinal nerve is the only one invariably present in the hind limb of mammals, while the nerve plexus may begin, according to the species, with any of the nerves from the twenty-first to the twenty-fifth, and, as it usually comprises iBve or six spinal nerves, it ends with the twenty-fifth to twenty-ninth nerve. It is thus probable that the hind limb readily shifts its position. As the sacrum is always developed in connection with the limb it follows that the number of prae-sacral vertebrae must vary, although there is no intercalation or obliteration of vertebrae.

Position of the Limbs

The primitive position of the limbs is at right angles to the body in a plane nearly parallel with the longitudinal body-axis. The first change is the appearance of two bends which give the limb the position which is permanent in amphibia; the bends are similar in the fore and hind limbs. The first bend (elbow or knee) is at the end of the upper limb (humerus or femur), and is such that the lower limb is flexed downward (ventralward) and toward the median line ; the second l>ond is at the carpus (tarsus) and is in the opposite direction or outward. Thus the ventral aspects of the forearms and lower legs come to look inwardly and their dorsal aspects outwardly; while the ventral asi>ects of the hands and feet look downward and their dorsal aspects upward. This change is obviously correlated with the change fn^m aijuatic to terrestrial life and the consequent substitution of legs for fins. When the position of the limbs has been no further altered than this, the radius and tibia are found on the cranial side, the ulna and fibula on the caudal side of their respective limbs. The second step is the torsion of the limbs, which is similar in both pairs and occurs in all mammalia, the result of which is that the digits point headward, the first digit being in both hind and fore limbs toward th(^ median line. This is the arrangement which is permanent in the rvptilia and in the lower mammalia. The torsion, by wliich the change is effected, does not take place in the arm or leg itself, but at the shoulder or hip. The third change is the torsion of the upper arm (not known to occur in the leg) by which the distal end of the humerus is twisted over through an angle in man of nearly or quite one hundred and eighty degrees ; by this torsion the head of the radius, which before the change was on the inner side of the arm, is brought across in front of the idna to the outer side, with the result that if the hand is kept in its primitive position, palm down, the forearm istwist(Ml in the reverse direction to the upper arm ; this third change is accompanied by accessor}^ modifications in the joints and muscles by which the radius l)ccomes so movable that it can l)e employed io turn t^gdmi^with the palm either up (supination) or down (pronation)

It is to be expeoteil ihiw tho limits of tlto hi>;hor )nan\n).M(UM iv»>s tlmtugh the three sta^^ of limb jx^^sition wliioli inav Iv i\>n\oinot\il\ designated Jis amphibiiui. x\^ptilian, and numunalian TnloiHi natelv there are no ol^fcservat ions as vet to slunv wluMbor tlnn \s ih«* case or not. This g-ap in our knowltHlp* otToi-s a faxorabloopjH^rln nitv for a resean^h.

Shoulder Girdle. — Tho anlago of tho shouldtT >;irdlo i> pi>»bid»lv continuous in all vertebrates, as it has Uvn shown IoUmu tho twlww, with the anlage of the Ivise of tho liniK but in tlio annuota it oarlx becomes a seiw rate cartihxgo, lying in one pI.Muoantl o\tondnl^:t^t^No ventrally. In mammals there is a largi* dorsjd sognn»nt o\ tliir* ww tilage above the articulation with tht^ humerus (gl«Muiid Inss^i) and ii much smaller segment Mow tho articuljititui. Tbodorsnl Mo^uirnt develops into tho large shoulder blaiKs while tho vrntrnl Mi*^nii*nl fomis merely the small coracoid proei»ss, ahht>ugh it is the hunm logue of the large and inde]H'ndent. cornooid In>ih« of Hnurti|if4idn and amphibia. It is to Ix) nottnl thnt Sabatier, 80.1, has htmiulo^'i/iMl the "coracoid" process with th(» pra»-onrar«>id, and holds that thn up})er third of tho mannnalian gliMioid fossa, which nHsiruM I'l-nni a separate centre, represents the true coracoid, but. Hhm \ ii'w ban nnt. lx»en accepted.

Little is known concr'rning tin* (htvi'lojirni-nt, nf thff urapidji in mammalia beyond what is giv«!n in \V. K. I'iirki-r'.'i nMino^riapli, 68.1, a work which has by no means n?ceived the attention it deserv'es. Owing to tho redur-tion of the cora^r<^>id in niarnmalia the history of \\u; sr-apida is practicfdly that of the erjtin* shoulder gird k-. Park<rr. i. r.. p. '-t'i'r-^'ii, r»-<^-'»rd-- roi/j" of/-< r va lions on the r'-ap d.i ',\' Ijuman embry.-. In ;j:. t-iiAfr.'f i't\ inch*— \'.:,'j. n.t' -'--ip'il;! '^y . \. ^ ^v„

C/% is smull and slightly curved; it is connected by a fibrous band with the end of the clavicle, hut the cartilaginous end of the clavicle (Parker 's so-called meso-scapular segment) is articulated by a synovial joint at this stage with the (?nd of the acromion. The coracoid has its own centre of ossification, to which are added at the time of puberty two epii)hysal centres (Rambaud and Renault) — its ossification thus indicating its morphological individuality. The acromion has two, sometimes three, centres, which appear between the fourteenth and sixte<.^nth years and soon coalesce, but the ossified acromion does not unite with the scapula until eight to ten years later. There is a separate centre fcjr the inferior angle (supra-scapular) and for the uj)i)er part of the glenoid cavity.

Clavicle

(Opinions differ as to whether the clavicle is a dermal bone or an integral portion of the scapular arch. It is, as discovered by C. Bruch, 63.1, >}7l-U'i, the first bone fomied in the human embryo its ossification going on during the st'venth week. Gegonbaur, has shown that the bone commences bv ossification of mesenchyma; then cartilaginous masses ai)pear at each end, which are, however, softer and have less basal substance than most embr3'onic cartilage; these cartilages serve to maintain the growth in length of the clavicle. KoUiker states (** Entwickelungsgeschichte," ISTU, p. -IIKO) that he has verified on rabl)it embryos Gegenbaur's observations, though he regards the tissue of the anlage as intermediate between mesenchyma and true cartilage. Kolliker adds that there is a separate centre of ossification, which may he compared to an epiphysis at the sternal end. This epiphysal piece was fii^st described by AV. K. Parker, 68.1, 223-*2'^4, and was shown by him to become distinct while still cartilage; Parker terms it the prsecoracoid, although this name is i)roi)erly applied to an entirely different bone. These peculiarities in the development of the clavicle, together with Rathke's stiitement that the clavicular anlage is at fii'st continuous with that of the coraco-scapular arch, and certain observations of his own, have led Alex. Goi'tte, 77.1, to maintain that the clavicle is an element of the shoulder. Goette's observations have been in part confirnunl by C. K. Hofmann, 79. 1. Gegenbixur regards the mammalian clavicle as a C(jmiK)und bone homologous with l)otli the true dermal clavicle (Decknoch(»n des Procoracoids) and the cartilaginous pro:H)racoid of fishes, the two originally separate skeletal elements having united with one another; by this double homology Gegenbaur explains the i)eculiar development of the l)one; compare his **Grundriss d. vergl. Analomie," '-Jte Aufl., 60 1 . It is possible, however, that we attribute too gn^at morphological meaning to the appearance of cartilage, and that jiartial chondrification of the clavicular anlaire d<K^s not mean, as Get^enbaur thinks, a separate element of the skeleton, or, as Goette thinks, connectirm with the shoidder girdle, but is menOy a modification of the histogenetic development — compare the paragraph on the mandible, p. 444. We cannot hojDe to understand the homologic^s of the clavicle until its development shall have been completely traced, begiiming with the earliest mesenchymal stage.

Episternum

Whether there is any epistennnn in the human embryo is uncertain. Perhaps the supnistenial cartilages just mentioned as having beeu descril^ed by G. Ruge, 80.1, are its representatives. K. Bardeleben, 79.1, has sought to hoinologize the deep portion of the interclavicular ligament as the rudiment of the human episternum. A. Goette, who has worked out, 77.1, the development of the parts more fully than any other anatomist, finds that "paired interclavicular elements grow out backward from the ventral ends of the clavicles, and uniting together form a somewhat T-shai)ed interclavicle overlying the front end of the sternum. This condition is jwrmanent in the Ornithodelphia except that the anterior j)art of the sternum undergoes atrophy. But in the higher forms the interclavicle becomes almost at once divided into three parts, of which the two lateral re^main distinct, while the median element fuses with the subjacent i)art of the sternum and constitutes with it the presternum (manubrifini stemf). If Goette's facts are to be tnisted, and they have been to a large extent confirmed bv Hofmann, his homologies apj^ar to be satisfactorily established."^ (Balfour.)

Pelvic Girdle. — The i^elvic girdle resembles the i>ectoral; it consists of a bar of cartilage which articulates with the femur; the articular cavity is known as the acetabulum and divides the girdle* into a dorsal and ventral segment, as the glenoid fossa divides the scapular arch. the dorsal pelvic division is called the iliac section, the ventral division the pubic section. The iliac section has no connection with the vertebral column in fishes, but is ai-ticulated with the sacral vertebrae in amphibia and amniota. The pubic section meets its fellow in the median ventral line; in amphibians it becomes more expanded and plate-like, and there appears an interruption of the cartilage by which the obturator foramen is formed ; this foramen divides iho pubic section into a cephalic portion or pubis, and a caudal ^)ortion or ischium. In mammals the foramen 18 enlarged so that ischium and pubis are more distinct than in amphibia.

Balfour ("Comp. Embryology," II., G()(>) found that the mcxle of development of the pelvic girdle in Scyllium is very similar to that of the pectoral girdle. There is a bar on each side continuous on its posterior border with the basal element of the fin (Figs. 345 and .'J47). This bar meets and unites with its fellow.

Concerning the early development of the girdle in amniota I know of only the observations of A. Bunge, wliose dissertation I have not seen, and those of Alice Johnson, 83. 1, on the chick. The latter shows that the girdle is contiiuious with the femur at first; the ischium and pubis grow out separately from the acetabular region, both growing ventralward, the former on the caudal, the latter on the cephalic, side of the crural nerve; if the ischium and pubis were to unite distally, which, however, they do not do in tin* chick, they would inclose a space homologous witli the obturator foramen. This observation renders it impn)bable that the ischium and pubis are together homologous with the i)ubic section <.)f the girdle in fishes, and indicates that one of them is a new element — added in the amphibia ^)erhaps. The pubis sends out a })rocess headward from just below the acetabulum; this process is the j)re-]nibis; it is well developed in the Omithorhynchus, but is rudimentary in the higher mammalia.

Skeleton of the Arm. — Our knowledge of the development of the skeleton of the fore limb in mammalia is verj* imperfect. It rests chiefly on the data furnished by Henke and Reyher, 74.1, supplemented by E. Rosenberg's valuable investigations of the centrale carpi in man, 75. 1, and a few observations recorded by Kolliker in his "Entwickelungsgeschichte," 2te Aull., and by C. Emery, 90. 1. To these references ought to be added one to the paper on the development of ungulate limbs by Alexander Rosenberg, 73.1, which, however, has less direct interest for us.

The skeleton of the arm in mammals (^is in amphibia also, H. Strasser, 79. 1) in its earliest mesenchymal stage forms an uninterrupted anlage (KoUiker, /.c, 401), with no indication of its future subdivision, and is, moreover, probably continuous with the anlage of the pectoral girdle. As soon as chondrifications begin the individual skeletal pieces are indicated by corresponding separate centres of chondrification, which begin near the centre of each piece and spread toward its periphery. The separation of each digital series is given in the primitive mesenchymal anlage, which also shows, according toC. Emery, 90.1, 206, traces of a sixth digit (prse-poUux) in front of the thumb ; the sixth digit persists as a rudiment and only for a short time. The condensed mesenchyma between two adjacent cartilages becomes fibrillar and produces the articulations. On the development of the joints, see p. 460. When, however, two cartilages fuse into one, as occurs in man with several of the carpals, the fusion takes place very early and no articulation is formed. It may be noted here that the joints are not differentiateil until six or eight weeks after chondrification begins.

In the human embryo at six weeks nearly all the skeletal pieces are present ; the ends of the humerus an» somewhat enlarged ; the ulna has a prix;essus anconaeus already ; the radius shows both head and neck; the metacarpals are beginning to chondrify. By the eighth week the phalanges are cartilaginous, having begun to chondrify (Kolliker, '*Entwickelungsgeschichte," 2te Aufi., 401) when the five digits l)ecame distinctly indicated by marginal notches in the hand, and in the humerus the calcification of the cartilage, preliminary to its degeneration and replacement by bone, has begun ; the articular surfaces of the cartilages are becoming more sharply defined (Henke u. Reyher, 74.1, 224-230). The.se authors discovered, /. c, p. 268, that the centrale exists as a separate structure in embryos of the second month. E. Rosenberg, 75.1, 172-101, has traced out the histor>' of the centrale very carefully; it is characterizeil by having less intercellular substance than the other carpal cartilages, and by never changing into bone, except as a ran^ anomaly; normally it is gradually absorbed in older embryos and disapi)ears, the space it occupied being taken up by the enlargement of the radiale (scaphoid) . Henke and Reyher have observed a tenth carpal also which was perhaps merely a transitory (Gegonbaur's *' radial sesamoid") bone — at least this suggestion <>f K. Rosenberg's is a plausible explanation.

Ossification. — "In the humerus a nucleus appears near the middle of the shaft in the eighth week. It gradually extends, until at birth only the ends of the bone are cartilaginous. In the first year the nucleus of the head appears, and during the third year that for the great tuberosity. The lesser tuberosity is either ossified from a distinct nucleus, which appeiirs in the fifth year, or by extension of ossification from the great tuberosity. These nuclei join together about the sixth year to form an epiphysis which is not united to the shaft till the twentieth year. In the cartilage of the lower end of the bone four separate nuclei are seen, the first appearing in the capitellum in the third year. The nucleus of the internal condyle appears in the fifth year, that of the trochlear in the eleventh or twelfth year, and that of the external condyle in the thirteenth or fourteenth 3'ear. The nucleus of the internal condyle forms a distinct epiphysis, which unites with the shaft in the eighteenth year; the other three nuclei coalesce to form an epiphysis, which is united to the shaft in the sixteenth or seventeenth year.

" The radixis is developed from a nucleus which appears in the middle of the shaft h\ the eighth week, and from an epiphysal nucleus in each extremity which only appears some time after birth. The nucleus in the carpal extremity appears at the end of the second year, while that of the head is not seen till the fifth or sixth year. The superior epiphysis and sliaft unite about the seventeenth or eighteenth year; the inferior epiphysis and shaft unite about the twentieth year.

" The ulna is ossified similarly to the radius but begins a little later. The nucleus of the shaft appears about the eighth week, that of the carpal extremity in the fourth or fifth year. The upper extremity grows mainly from the shaft, but at the end of the olecranon a small epiphysis is fonned from a nucleus which appears in the tenth year. This epiphysis is united to the shaft about the seventeenth year; the inferior epiphysis about the twentieth year.

From what is stated al)ove it appears that in the bones of the arm and forearm the epiphyses which meet at the elbow-joint tegin to ossify later, and unite with their shafts earlier, than those at the opposite ends of the bones ; whereas in the bones of the thigh and leg the epiphyses at the knee-joint are the soonest to ossify (except in the fibula) and the latest to unite with their shafts. In the lK)nes of the arm and foreann the arterial foramina are directed toward the elbow; in those of the thigh and leg they arc directed away from the knee. Thus, in each bone the epiph^'sis of the extremity toward which the canal of the medullary artery is directed is the first to be united to the shaft. It is found also that while the elongation of the long bones is chiefiy the result of addition to the shaft at the epiphysial synchondroses, the growth takes place more rapidly, and is continued longer, at the end where the epiph3'sis is last united ; and the oblique direction of the vascular canals is due to this inequality of growth, which causes a shifting of the investing i)eriosteum, and so draws the proximal portion of the medullary artery toward the more rapidly growing end.

" The carpus is entirely cartilaginous at birth. Each carpal bone is ossified from a single nucleus. The nucleus of the os magnum appears in the first year; that of the unciform in the first or second year; that of the pyramidal in the third year; those of the trai)eziuin and the lunar bone in the tifth year; that of the scaphoi in the sixth or seventh year; that of the trai)ezoid in the seventh or eighth year; and that of the pisiform in the twelfth year.

'*the metacarpal bone,s and plialauijes are usually formed each from a principal centre for the shaft and one epiphysis. The ossification of the shaft l)egins about the eighth or ninth week. In the inner four metacarpal bones the epiphysis is at the distal extremity, while in the metacarpal bone of the thumb and in the phalanges it is placed at the proximal extremity. In many instances, however, there is also a distal epiphysis visible in the first metacarpal bone at the age of seven or eiglit years, and there are even traces of a proximal epiphysis in the second metacaq)al. In the seal and some other animals there are always twoepiph^'ses in these l)onc»s. The epiphyses begin to l>e ossified from the third to tht* fifth year, and are united to their respective shafts about the twentieth 3"ear. The terminal phalanges of the digits i>resent the remarkable pecadiarity that the ossification of their shafts commences at the distal extremity, inst(?ad of in the middh? of their length, as is the case with the other phalanges and with the long bones generally (F. A. Dixey)." (O. I). Thane in Quain's Anat./' tenth edition.)

Skeleton of the Leg

The primitive mesenchymal anlages of the skeleton of the leg, like that of the arm, is continuous throughout in amphibia, H. Strasser, 79.1, and birds, Alice Johnson, o5.1, and therefore probably in mammals also, and in birds it is continuous also with the jielvic girdle, which appears as an outgrowth of the skeletal aidage of the limb proper. As in the arm chondrification bl(X*ks out the separate skeletal pieces. Tlu^ formation of cartilage begins in the chick the sixth day and becomes well marked by the seventh day, when Strasser's '* prochondral elements," p. 404, have already disappeared (Johnson, 1, c).

In the human embryo at six weeks all the skeletal parts are mapped out in cartilage, exc(^pt the terminal phalanges, which are still entirely mesenchymal. 11 le plan of structure is essc»ntially the same as in the arm at the same age, but the differentiation is less advanced; the femur has already neck and trochanter, is slightly curved, and its lower end is enlarged, with two condyles and the incisura intercondyloidea recogniziible ; the tibia has broad condyles at its upper end and is suddenly restricted immediately below, and slowlv increases in diameter toward the tarsus, to end with a surface so oblifpio as to bo nearly parallel with the length of the limb; the astragalus (talus) consists of a lower main portion, the homologue of the tibials, and an ujjper pnx*ess lying l)et\vcMMi the tibia and fibula, and homologous vr\\\i the intermedium; the fibulare (calcaneum) is not so long as the astragahis, and is separatcnl i)y articidar mesenchyma from both the libuhi and astragalus, alongsi<le of which last it is situated, but this situation is found to alter gradually, l)eginning to alter in embryos but little over six wei'ks. In the digital ravs the metatarsi and first phalanges only are differentiated (Henke^and Reyher, 74.1,-i:)0-*.':J4).

In an embrvo of nearlv six months the ankle has, I have found, essentially the adult form. As shown in a vertical section, Fig. "^h\y

the lower ends of the tibia, 7V>, and filnila, Fh, are still cartilaginous; the astragalus, .4.s7/', and calcanenm or os caleis, Cal, are wholly cartilaginous, although penetrated by vessels preparatory to their later ossification. The astragalus, .I.s7/-, is in quite different relations from thosi^ found at six weeks; it underlies the til)ia, and shows clearlv the suixlivision of its tibial articulation into tht' joint witli the main shaft, Tb, and willi the internal malK»olus, tfi : ))v its external surfa(;e it articulates with the fibula, Fib; by its lower surface witli tlu^ os calris, CaL All of these articulations are well differentiated. At its lower internal angle the cartilagi* of tli(^ astragalus is interrupted to allow the irrui>tiou of the vascular mc^scnchyma.

OssFFKATioN. — " Tho feniitr is developed from one princij)al (jssific centre for the shaft which appivirs in the seventh w(H'k, and from four e[>ij)liyscs, the centres for which appear in the f( ►Rowing order: A siiigh^ nucleus for the lower extremity ai>pears shortly iK'fore birth, one for the head apixvirs in thetirst year, one for the great trochanter in' the fourth venr, and one for the small trocliant<T in the thirteenth or fourteenth year. These ei)iphyses become uniteil to the shaft in an ord(^r the reverse of that of their aj)pearance. The small trochanter is imited about tin* seventiH^nth year, the great trochanter about the eighteenth year, the head from the eighteenth to the nineteenth vear, and the k)w<'r extremitv soon after the twentieth vear. Th(» neck <»f the f.'miir is formed bv extension of ossification from the shaft.

"the tlbin \\\\(\ Jibnhi t'ach ])reseiit, besiiles the ]>rinci])al centre for the shaft, a su|)erior and an infrriwr epiphysis. In the tibia the centre for the shaft apj;ears in the seventh week; that f<»r the u])|)er (extremity including both tuiierosities and the tubercle, apj>ears most friMjuently before, but sometimes afti'r birth ; and that for the inferior extremity and internal malleous a])pears in the second y(»ar. The tubercle is occasionally formed from a separate centre. The lower e]>i|)hysis and shaft unite in the eighteenth or ninetecMith year, th<' upper «'])iphysis and shaft in the twenty-first or twenty-second year. In the tibula the crntre for the shaft appears rather later than in the tibia; that for the lower <»xtremity a])pears in the second year, aiid that for the upjMT. unlikt* that of the tibia, not till the third or fourth year. The lower epiphysis and shaft unite alnnit the twenty-lir>t year, the u|»per ej)i]>hysis and shaft about the tw(»ntyfourth vear.

" The tarsal bones are ossified in cartilage, each from a single nucleus witii the exception of the os calcis, which in addition to its proper osseous centre has an epiphysis upon its posterior extremity. The principal nucleus of the os calcis appears in the sixth month of foetal life; its epiphysis begins to be ossified in the tenth year, and is united to the tuberosity in the fifteenth or sixteenth year. The nucleus of the astragalus appears in the seventh month ; that of the cuboid about the time of birth ; that of the external cuneiform in the first year; that of the intenial cuneiform in the third year; that of the middle cuneiform in the fourth year, and that of the navicular in the fourth or fifth year.

"The metatarsal bones sxidphalanges agree respectively with the corresponding bones of the hand, in the mode of their ossification. Each lx)ne is formed from a principal piece and one epiphysis; and while in the four outer metatarsal bones the epiphysis is at the distal extremity, in the metatarsal bone of the great toe and in the phalanges it is placed at the proximal extremity. In the first metatarsal bone there is also to l>e observed, as in the first metacarpal, a tendency to the formation of a second or distal epiphysis (A. Thomson) . In the metatarsal bones the nuclei of the shafts appear in the eighth or ninth week. The epiphyses appear from the third to the eighth year, and unite with the shafts from the eighteenth to the twentieth year. The nuclei of the shafts of the phalanges appear in the ninth or tenth week. The epiphyses appear from the fourth to the eighth year, and unite 'W'ith the shafts from the nineteenth to the twenty-first year." (G. D. Thane in Quain's Anatomy," ninth edition.)

Joints of the Limbs

Our knowledge of the development of the joints is biised chiefly upon the researclies of Henke and Reyher, 74.1, Bernays, 78.1, and Hepburn, 89.1; Hagen-Tom's article, 82.1, is chiefly on the histogenesis of the synovial membrane, see p. 421. Where a joint is to be formed the cells l)ecome elongated at right angles to the axis of the anlage {synnrthrodial stage) ^ the tissue becomes fibrillar and in its midst the cavity appt»ars {diarthrodial stage) ; chondrification soon extends to the cavity, the articulating surfaces thus l)ecoming cartilaginous. The development of the joints is very gradual, but by the end of tin* third mouth there are true articulating surfaces, which gradually bec*ome better developed; the development of the joints progressc^s distally, thus the elbow-joint is developed much earlier than the finger- joints ; the articulations of the arm appe^ir sooner than the corresj)onding ones in the leg, thus the knee-joint ap|)ears later than the elbow- joint. Bernays, 78. 1, stiites that the s^Tiarthrodial stage of the knee lx»gins in a human embryo of 2 cm., ami still i)ersists in one of 3 cm.; in the latter, althougli there is still no articular cavity, yet the articular ends of the femur and tibia are shaiied nearly as at birth — an important olxservation because it shows that the articulating surfaces are shaped l)efore any free motions can l^egin. In the three-centimetre embryo the growth of the lateral tibial crondyle has already forced the fibula t>ut of its intimate connection with the femur, which is characteristic both for the earlier stage in man and for ancestral types. By comj^arative anatomy Bernays has sought to prove that svnarthrodial joints are characteristic of the fishes, imperfect diarthrodial joints of the amphibia, perfect ones of the amuiota. Hepbum, 89.1, adds but little to our knowledge, but his paper is valuable for an admirable synopsis of the stages of joint differentiation and of the classification of joints from the embryological standpoint. Hepburn's classification is essentially as follows: Syndesmosis, synchondrosis, primitive articular cavity, amphiarthrosis, diarthrosis (simple, double with meniscus) ; the diarthroses show the following stages : 1 , surfaces become cartilaginous ; H^ capsular ligament formeil ; 3, other ligaments formed ; 4, s\Tiovial membrane developed.

III. Dermal Bones

It has been long known that not all the bones are praeformed in cartilage, and that some of them, especially of the head, are developeti from soft tissue. The latter were known to the older anatomists as membrane bone.s. In the years 1845-50 the origin of the membrane bones was actively debated, and at that time the term secondary bones was substituted for the earlier designation, and the termB Belegknochen and £>ecA:A:/*ot7i<^/i were introduced by KoUiker, whose investigations played the principal part in demonstrating that the membrane bones are developed by the direct ossification of young connective tissue, or — as we should now say — of mesenchyma. Those who wish to follow this discussion are referrtnl to KoUiker, 60.2, where references are given to various authorities of the time, and also to Kolliker's '* Bericht dor Zootom. Anstalt in Wiirzburg," and his "Entwickelungsgeschichte," '^te Aufl., 403. The dermal bones of the head may lie close against the cartilage (or bone) of the primordial skull, and in that aise are often calleil splint bones or splenial bones.

In the lower vertebrates the niembrano bones acquire a greater development than in higher foniis, and in certain ganoids and teleosts are develope<l over nearly the entire body, whereas in the amniota they are confined to the head.

O. Hertwig's brilliant researches, 74.1,2, 76.1, 79. 1, have demonstrated that the dermal boiK»s are homologous with the plates formed by the fusion of epidermal tin^th or so-called placoid scales. The placoid scales are true teeth develoi)ed in the skin and supported by a base of bone ; by the fusion of adjacent bon}- bases we may have an osseous plate develoj^d in the cutis. In tailed am})hibia several of the membrane bones arise* as <lentiferous plates, but later in the development the teeth are res()rl)ed leaving merely the lx)ny plate, but in anoura the homologous lx)nes are develo{)e<l without teeth being fonne<l at all. The inevitable conclusion from these facts is that the dermal skeleton has Ix^en evolved through three principal stages: 1, scattered iiKlei)eiident <lennal teeth (placoid scales); 2, teetb-bearing plates formed by the fusion of the exj)anded bases of adjacent teeth (exo-skeleton) ; 3, inombrane bones developing without teeth apijearing (dermal Ixmes of tailless amphibia and amniota).

The plates or bones of the dermal skeleton are not the same throughout the vertebrate series ; among the fishes there are numerous modifications, the homologies of which have not j'et been thoroughly elucidated ; in the amphibia we encounter all the elements of the dermal skeleton of the amniote hetid, and comparative anatomists have succeeded in homologizing some of these elements with plates in fislios, but as mucrh remains to be done, and as the conclusions have not hitherto been based ujxm much embryological evidence, I shall not attcjinpt to enter into these ditficult discussions.

Typical Dermal Bones of Amniota. — In amniota the dermal bones are confiueil to tlje skull and fa<'e. There are, 1, four i)airs of bones on the dorsal side, namely, the nasals overlying the olfactory chambei-s; \\io frontals overlying the anterior part of the brain cavity; the pftriefals ovtM-lying the middle part of the brain cavity, and the interj)ari(*ta/s overlying the anterior part of the occipital region; the frontals, pari(?tals, and interparietals, together with the supraoccipital, constitute th(3 roof of the skull; wlien the cartilaginous skull spreads u])ward it g<x»s uniler the territory of the frontals, parietals, and interparii'tals, and when it ossifies it may contribute to a greater or less extent to the bones in question, so that they are not excrlusively membranous in origin (L)nrsy). Between the parietals and supra-CKrcijiital is the /// trrparirfal: '2. Tiie small l(fch rt/iiutls situated lx»twe<'n tiio n:isals, fronbds, and the eye on each side (in certain reptiles there are additional |»erior])ital bones), and the sqnaiiutsal^ occupying the sj)ace l>etwei?n the parit?tals, ali-sphenoids, and occipitals, and overlying that i>ortion of the man<libular bar which forms the quadrate of reptilia (incus of mammalia) ; the squamosal is [M?rhaps the homologue of the pne-oix.»rcular of fishes, as maintainenl by Huxley, or perhaps of the ganoid supratemix:)ral as suggested by Balf»»ur, '^Comp. Embryol.," II., o03. :>. The bones associated with the mandibular branchial bar; these are, first ^ those asS(X*iated with the palato-quadrate b<irs and ap|>earing in the roof of the mouth, the romrr, palatines, and ptenjiiaids: second, a series associated with Mec^kel's cartilage, and consisting j)rimarily, according to comparative anatomists, of thi*ee dermal Imjucs, the distal dentale, the smaller artirularr, an I in the angle bt^twtH'U these two the small an(ptlare: but in mammals there is only a single lx)ne developtnl from the* mj-ien(»hynia ciround Meckel's cartilage, which evidently represf^nts tin* dentaU*, but whether or not it also represents the articulare and angidare has not been definitely Si'ttled. 4. The series associated with the maxillary processes, four on each side forming a row : l)eginning at the ventral end of the ])roc«^ss these four Ijones are the pnp-nia.cilla, nHi.rilla, Jugah and qmulratn-jiKjal. .">. Tiie median para-sphennid, which is developed in tlu» roof of the mouth in many fishes (but not in elasniobranchs or mMrsi|H)branchs), in amphibia, an<l in sauropsida, in which last it is less iniixn'tant and becomesindistinguishably fusinl with the sphenoid in the adult: in mammalia it has not lx?en found, though probably morphologically presf»nt in the sphenoid — a probability which it would 1h» wortli testing by a sj>ecial investigati<»n. t;. The tiimpiimd Imne formed aroimd the drum of the ear.

The Dermal Bones in Man. — The numerous dermal bones. mentioncil as characteristic for the anmiota at large, have all Invn identifies! in the adidt human skull, ext*ept the artimdare, angulan\ quadrato-jugiilar, and para-sphehoid. The four bones mentioned are, however, all probably represented by definite parts as follows : the interparietal by the upper median portion of the supra-occipital ; the articulare and angulare by parts of the adult mandible; the auadrato-jugular by one of the ossificatory centres of the jugal, and the para-sphenoid by part of the sphenoid. The nasals, parietals, lachrymals, vomer, and jugal remain independent bones, while the frontals and palatines are also independent except that each pair forms but a single bone. On the other hand the squamosals, pterygoids, dentals, are united with certain parts of the primordial skull. Finally the prae-maxillaries and maxillaries fuse into a single bone, of which the part bearing the four upper incisors corresi)onds to the prae-maxillaries.

A tabular view of the homologies of the human skull is given on p. 4G5.

The following data afford additional information concerning the development of the single dermal bones.

Nasals are each ossified from a single centre which appears about the eighth week.

Frontal is ossifie<l from two centres, one for each frontal appearing about the seventh week. At birth the frontals are still entirely distinct, but they become united during the first year after birth by the median " frontal" suture, which usually becomes obliterated by osseous imion taking place from below upward during the second year, but not infrequently the suture j)er8ists throughout life.

Parietals are each ossified from a single centre which appears in the site of the parietal eminence about the si^venth wc»ek. The eminence is very conspicuous in the young lx>ne and gives a marked character to the form of the skull for a number of years in early life.

Interparietals are represented by the upixir pair of centres of the supra-occipital region; these centres appear during the seventh week in the mesenchyma overlying the supra-occipital cfirtilage. The interparietals usually unite with the true occipitals, but occasionally they remain distinct and are then separated from the supra-occipital by a suture running transversely from one lateral angle of the occipital bone to the other.

Lachrymals are each ossified from a single centre, which appears al>out the eighth week.

Squamosals are each ossifie<l from a single centre, which appears in its lower part about the seventh or eighth wfH^k ; ossification spreads upward in the s^juamosum pro|x»r, and outward into the zj'gomatic process. At birtli the s<iuamosal is still separates! from the periotic capsules, but during the first year lK)ny union is effected and the squamosal becomes a part of the tonifx^ral Ixjne of the adult.

Vonier is ossifif*d from a single nucleus appearing at the hinder part alKHit the eighth woek. From this nucleus two laminae are develop(Ml, which pass ui>on either side of the nK^lian line and embrace the lower part of the cartilaginous intemasal sf»ptum. These laminae gradually ooal(»sce from l>f*hind forwanl till the age of pulK*rty, thus forming a mesial plate, with only a groove remaining on its sujierior and anterior margins.

Palatine is ossified from a single centre which appears in the seventh or eighth week at the angle between its horizontal and ascending parts.

Pterygoids are each ossifie<l from a single centre which appears during the fourth month; during the fifth or sixth month the pterygoids unite with the ossified pterygoid processes (future external pterygoid plates) of the ali-sphenoids and thus become the internal pterj'goid plates of the adult basi-sphenoid.

Prce-maxiUaries have Ijeen studied by Th. Kolliker, 82.1; they ossify later than the maxillaries and appear just before the palate fissure closes, and after the fissure lias closed they are found united with the maxillaries so that the period of their independent existence is very short; but in the ninth week traces of the primitive division are still present, and even these traces disappear by end of the tenth week. The pne-maxiUaries csrry the four upper incisors. A special interest attaches to these bones because their homologies in man were ascertained bv Qoette.

Maxillaries begin to ossify toward the end of the second month and offer the peculiarity of starting from several spots, which, however, speedily fuse and cannot l)e regarded as separate centres. This peculiarity was first recorded by Beclard, 20.1, and his observation has been confinned by Kambaud et Renault, and more recently by Callender, 70.1, 10.5. As stated above, the maxillaries and prae-maxillaries are united before the tenth week.

Jngals^ or malars, begin to ossify about the eighth week. According to Rambaud et Renault, ossification begins from three points, which are found united by the fourth month.

Mandible. The mandible of the adult is a compound bone, for it includes both the dermal bone and the ossified lower ends of Meckel's cartilage, most of which, however, is resorl)ed, and it is further peculiar in having cartilage develoi)ed at the ends of both the corouoid and condylar processes. The two mandibles are distinct at birth, but during the first year their lower or ventral ends unite, but in a pig embryo of two and a half inches Parker (*' Morphologj' of the Skull/' 200) describes the ends of Meckel's cartilages as united, and it is probable that the cartilaginous jaws of the human embryo are similarly united. The development of the human mandible has been studied by Mas<iuelin, 78.1; in an embryo of 5 cm. the cartilage of Meckel is entirely surrounded by mesenchymal bone, and in embryos of 17 cm. there are only slight c^dcified remains of the cartilage, except in the lower ends near the symphysis, where, as shown by Kolliker, the cartilage participates in the ossification of the mandible; the cartilage of the coronoid process was found in embryos of 7.5 and 0.5 cm., and in the later cartilage along the alveolar border ; the cartilage of the condyle is developed still earlier ; the three cartilages upon each mandible undergo direct ossification. Strelzoff, 73.1, was led by the observation of these cartilagt^s to maintain that the entire jaw is preformed in cartilage, but that this view is erroneous was demonstrated in an admirable paper by J. Brock, 76. 1. It is evident that the accessory cartilage of the mandible is morphologically distinct from that of the primordial skeleton.

Tympanals develop during the third month oach from a centre which appears in the lower part of the external membranous wall of the tympanum and extends upward xmtil a nearly complete bony ring is formed, inclosing the tympanic membrane; liefore birth the ends of the open ring become united with the siimmiosiiU and thus incorporated in the great temporal bone of the adult.

The Fontanelles

These are membranous intervals between the incomplete angles of the parietal and neighboring bones, in some of which movements of the soft wall of the cranium may be observeil in connection with variations in the state of the circulation and respiration. They are at the time of birth six in number, two median, anterior ana posterior, and four lateral. The anterior fontanelle, situated between the antero-superior angles of the parietal bones and the superior angles of the imunited halves of the frontal bone, is quadrangular in form and remains open for some time after birth.. The posterior fontanelle, situated between the posterosuperior angles of the parietal bones and the superior migle of the occipital bone, is triangular in shape. It is filled up before birth, but the edges of the bones being united by membnme only are still freely movable upon each other. The lateral fontanelles, small and of irregular form, are situated at the inferior angles of the pariet^d bones. The fontimelles are gradually filled up by the extension of ossification into the membrane which occupies them, thus completing the angles of the bones and forming the sutures. The closure, especiaUy of the posterior and lateral, is often assistinl by the development of Wormian bones in these situations. All traces of these unossified spaces disappear before the age of four years.

IV. Morphology of the Skull

We are now in a position to consider several questions concerning the skull as a whole. What is presented on these (luestions I have divided under the following headings into sections: 1. Homologies of the boftes of the human skull. 2. Relations of the primary and secondary' skull. 3. Position of the facial apparatus. 4. Significance of the trabeculsB cranii. 5. Theories of the skull. The detailed histor}'^ of each element of the skull is given, as fully as practicable, in the preceding pages.

Homologies of the Bones of the Human Skull. — These have been discussed in the preceding pages of this chapter, but it w^ll be convenient to present the conclusions arrived at in a tabular form:

Relations of the Primary and Secondary Skull

Comparative anatomy and embryology alike teach us that we must attribute to the skull a double origin, or rather that there are two skulls, one outside the other, which were i)rimitively distinct from one another, but in the progress of evolution from the earliest fish tj'pe to the higher mammalia the union between the two skulls becomes more and more intimate. The inner skull is what is known as the primordial skull, with which I include the branchial skeleton; the outer skull comprises the series of dermal bones of the cranial and facial regions.

The primary skull appears first as the continuation into the region of the head of the axial mesenchjTual skeleton, which in the neck and rump is the anlago of the vertebne. That the meseuch3'mal skull represents in part, at least, a series of vertebrae is certain, and we find it sending dorsal outgrowths to inclose the brain just as the true vertebrae cover in the spinal cord. The mesenchymal skull also extends in front of the liyix)physi8, where it provinces the trabecular cranii. What little can Iyg surmised concerning the original homologies of this pai*t of the skull is ppven in the section on the trabecuhe, p. 4:U. The mesenchymal skull grows so as to completely incase the brain and partially incase the olfactory chamlx^rs. While it is growing six centres of chondrification aj)j)ear in it : namely, two trabecular, two parachordal, and two pt^'iotic; each centre forms a cartilage, which is extraordinarily uniform in shape and relations throughout the entire vertebrat^^ series; the six cartilages remain distinct for a very short time only; the two tral>c»cuLne unite first, the two parachordals next, third the unittni paracliordals (or occipital) coalesce with the jwriotic capsules and later with the caudal ends of the trabecuke, thus forming a largo Hoor of cartilage under the brain. In the lower forms chondrification spreads until the entire primary skull becomes cartilaginous, and it is in this condition we find the skull in many of the fishes.

In the amphibia and amniota there is a progn^ssive reduction of the cartilaginous skull by which its development as a nxif over the ])rain is more and more diminished. This reduction leav(»s an ojxming as it were on the dorsiil side, and at once increases the impK)rtance of the covering dermal lK)nes — fn>ntals, parietals, and interparietals. In Sjiuropsida tin* oix»ning is larger than in amphibia, and in the mammalia tliore is furtluT progn^ssive increase in size, as shown by Parker's olwervations, the oix^ning lx>ing larger in pigs than in insectivoni an<l e<lentntt»s. In mammals there is a further loss, which is not found in other classes, namely, an absence of chondrification in the region Ix^tween the ali-sj)henoiils and ix^riotic capsules, by which the importance of the scpiami^sal — the dermal bone of the region — is increased; see W. K. Parker, 86.1, 8, who speaks of the disappearance of the cartilage under the squamosal as *'the true diagnostic mark" of the mammalian chondrocranimn. Reduction of the cartilages of the branchial skeleton also progresses from the lower to the higher vertebrates. This shows itself in mammals not only in the total disappearance of the cartilages of the fourth and fifth arches, but also in the partial disappearance of the thyro-hyoid bars and the imperfect development of the hyoid bars. It shows itself f iui;her in the reduction of the mandibulars, for not only is the greater part of Meckel's cartilage resorbed as in all amniota, but also the palato-quadrate is very much reduced. As the palato-quadrate is an important part of the skull in the amphibia, the palatines and pterygoids appear as true splint bones, whereas in mammalia they have greater independence. It is clear from the above that the evolution of the mammalian skull has depended to a large extent upon the reduction or partial degeneration of the inner skull, or primordial chondrocranium.

The secondary or outer skull is not so old as the inner skull, and originated in the higher fishes as a series of dermal bony plates, which overlaid the primary skull, and probably formed a nearly complete case for the head, including the face. The definite arrangement of the plates, as perpetuated and modified in mammalia, appears in the amphibia, and was perhaps evolved during the transition from the fish to the amphibian type. The dermal plates (membrane bones) may either remain as splint-bones, as for instance is the case with the vomer, or they may coalesce with the underlying portions of the chondrocranium, as for instimce occurs with the interparietals in primates, or they may remain where the cartilage disappears beneath them, as for instance the frontals. Already in the amphibians the co-ordination and fusion of the inner and outer skulls into one complex skull is established, and in the amniota the welding together is carried still further, and the elements of the outer skull, i,e, the deiTnal bones, acquire increased importance as the inner skull, i,e, chondrocranium, is reduced. In brief, the evolution of the mammalian skull has depended largely upon increased morphological prominence of the dennal bones.

If we designate the formation of the chondrocranium as the first stage, and the formation of the dermal bones as the second stage in the evolution of the skull, we may designate the ossification of the primordial chondrocranium as the third stage. As to what caused that ossification, we have not even an hj'pothesis, and we are equally in the dark as to how the number of separate bones, or centres of ossification, was determined. It is noteworthy that the number of the primordial l)ones is extraordinarily constant.

Finally, let me emphasize the fact that, given the full number of bones, there is a sustained tendency to reduce them by fusion. The numl)er of skull tones is less in the amphibia than in the teleosts, in edentates than in amphibians, in man than in edentates. ^ thorough comparative study of the number of the skull bones is much to be desired.

Position of the Facial Apparatus. — Owing to the head-bend of the embryo, the oral invagination, or mouth cavity, is brought between the fore-brain and the lieart, and upon the ventral surface, and this is the permanent position in the sharks. If we follow through the vertebrate series, or the development of an amniote, we find in either case a steady increase in the region of the olfactory and oral invaginations, in consequence of which it projects more and more, and further by a throwing of the whole head upward the face is brought forward and projects in front of the brain. In man this c^)ndition is again modified : firsts because the upright position renders it unnecessary' to bend the head as in quadrupeds, and, therefore the head is left facing ventralward; secondy because the enormous development of cerebral hemispheres has rendered an enlargement of the brain cavity necessar}% and this enlargement has taken place by extend iiig the cavity over the olfactory regions as well as by enlarging the whole craniiun; thirds because the development of the facial apparatus is arrested at an embryonic stage, the production of a long snout being really an advance of development (Minot, 35), which does not take place in man.

I consider it not improbable that the axial jjerichordal mesenchymal skeleton sends an outgrowth past the hypophysis to inclose the fore-brain, and that, assuming that the infundibulum marks the true anterior limit of the medullary canal, the trabecular anlage is not a prolongation of the fl(X)r of the cranium, but an upgro\\i:h, which owing to the head-bend has come to lie in the line of the cranial axis.

Theories of the Skull.— It was noticed a long time ago that the skull has resemblance to vertebne; the skull has the greatest thickness yn the ventral side of the brain and arches over the central nervous system, and thus possesses two of the chief characteristics of the vertebrae. It was, therefore, natural to seek to comi)are the skull homologicall^^ with vertebne. It is said that during the eighteenth century this comparison acquired greater prominence and was definitely formulated by Vicq d'Az^'r.* These comparisons of Vicq d'Azyr and others procee<led upon a false basis, and it was not until 1872, when Gegenbaur, 73.1, opened an entirely new method of solving the morphology of the head, that correct views began to be formed. Another great stride was made by Froriep's observations on the development of the occiput, p. 4'20. I have placed what I haye to say under the three headings of Vicq d' Azyr's theory, Gegenbaur's theory, and Froriep's laAv.

1. Vicqd'Azyr's Theory.— According to this theory the skull consists of several vertebrae. Whether d'Azyr really originated it,

I have not suooeeded in finding anything in Vioq d'Axyr's ^'CEuvres" to justify this statement.

I cannot say. It was taken up by Oken, who is often quoted as the founder of it, and later also by Goette, who by some authors has been cited as the father of the theory. The history of the theory and of the modification it underwent is given by R. Virchow (" QoettQ als Naturf orscher ") .

One of the earliest suggestions of the vertebral theory is that of Burdin, independently made about the same time by Heilmeyer. These authors compared the skull to a single complex vertebra. Oken conceived that there were four cranial vertebrae, and this was the notion most in favor until 1858. Goette counted six vertebrae, of which three belonged to the facial apparatus. As to the number of thea^ supposed vertebrae there is a very extensive literature, which possesses an interest purely historical. Let it suflSce, therefore, to state aphoristically that three vertebrae were advocated by Spix, Meckel, Burdach and Cams; four by Oken, Bojanus, and Owen ; six by McClise ; seven by Geoflfrey .

The death-blow to this long-lived error was dealt by Huxley in his Croonian lecture delivered in 1858, 58. 1 — a great achievement, for it at once terminated the history of ^the old vertebral theory of the skull, and paved the way for Gegenbaur.

GEGENBArR's THEORY. — In 1872 Gegenbaur published his great work, 72.1, on the cephalic skeleton of Selachians, in which he took the ground that the skull does not represent a series of vertebrae, but that it arose out of the axial or i>erichordal skeleton before distinct vertebrae were formed in the axial region ; he further maintained that the head includes a number of segments, which he sought to ascertain by determining the segmental arrangement of the cranial nerv^es. This was a great step and in the right direction. F. M. Balfour, 78.3, was, I believe, the first to endeavor to trace out the actual number of segments (mesoblastic somites) in the head of embryos. A vast amount of labor has been expended by subsequent writers in investigating the development of the cephalic myotomes and cranial nerves, but much remains to be done before the morphological constitution of the head shall be understood, but we are already in a position to say that Gegenbaur's thesis — that the primary or inner skull is developed from the axial skeleton but not from vertebrae — is correct except as regards the hypoglossal region. For further observations on the segmentation of the head see Chapter XXVI.

Froriep's Law. — Froriep's investigations, p. 429, have demonstrated that the skull has extended itself, in the amniota at least, by the annexation of true vertebrae, corresponding to segments of which the hypoglossus represents the nerve. The head has grown at the expense of the neck.

Present Theory of the Skull. — The primary skull was developed out of the axial (perichordal) skeleton, in the region of the brain, where the dorsal and ventral nerve roots are not united into a single ner\'e for each segment ; the primary skull has grown at least in the amniota by the annexation of several cervical vertebrae; a secondary skull was developed outside the primarj^ cartilaginous skull by the formation of dermal bones. In the higher forms the primary skull partly disappears ; what remains, together with the secondary or dermal skull, constitutes the actual skull of the adult.

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